Why do we rest?

You will spend approximately a third of your lifetime laying horizontally on a mattress, resting your head on a soft pillow, covering your body with a soft blanket or two, with your eyes closed. Whilst you are resting on your bed, your entire body is at rest relative to the Earth's surface, and the rest of the bed you're not sleeping on will become colder. For people living in a Kansas town, every night they rest in Rest. If your arm rests on an armrest, your feet rests on a footrest, then does your body rest on a bodyrest rather than a bed? 

Thoughty2 videos about sleep: 
https://www.youtube.com/watch?v=P3KQ5p8C8FA&ab_channel=CentreInternationaldeRencontresMath%C3%A9matiquesCentreInternationaldeRencontresMath%C3%A9matiques

https://www.youtube.com/watch?v=rVoQoAbVslY&ab_channel=Thoughty2

https://www.youtube.com/watch?v=6OHYUiav0RQ&ab_channel=Thoughty2

https://www.youtube.com/watch?v=FmM3YZQrIWA&ab_channel=Thoughty2

https://www.youtube.com/watch?v=_RSdGRQP9ZA&ab_channel=Thoughty2

QI videos about sleep: 
https://www.youtube.com/watch?v=KUQVoJ7rtME&ab_channel=QI

https://www.youtube.com/watch?v=p_Ih55x_Sc4&ab_channel=QI

https://www.youtube.com/watch?v=xUXfWADMPSE&ab_channel=QI

It's Okay To Be Smart video about sleep: 

https://www.youtube.com/results?search_query=sleep+it%27s+okay+to+be+smart

What is a bed? 

https://en.wikipedia.org/wiki/Bed

A bed is defined as a piece of furniture used for the purposes of sleep, relaxation, or engaging in sexual activity. Most modern beds consist of a soft, cushioned mattress on a bed frame, with the mattress sitting either on a solid base, wooden slats, or a sprung base. Beds typically include a box spring inner-sprung base, which is a large mattress-sized box consisting of wood and springs that provide supplementary support and suspension for the mattress.

When was the bed invented? 
(a) Ancient history 

• The earliest beds were thought to have made out of piles of straw or an unnatural material (e.g. a heap of palm leaves, animal skins, or dried bracken). Then those early beds were raised above the ground to avoid drafts, dirt and pests. 
• A 2014 National Geographic News report estimated about 23 - 5 million years ago, ages began to design beds composed of a sleeping platform including a wooden pillow. Wadley et al. (2011) estimated the date of the bedding discovered in Sibudu Cave, South Africa to be approximately 3600 BC, which consisted of sedge and other monocotyledons coated with the leaves of Cryptocarya woodii Engl. 
• Beds discovered in a preserved northern Scottish village were raised boxes made of stone and possibly topped with comfortable fillers. They were dated to between 3200 BC and 2200 BC. 
• The Egyptians made high bedsteads ascended by steps, with bolsters or pillows, and curtains hung around. It's theorised the elite of Egyptian society such as its pharaohs and queens slept in wooden beds, occasionally gilded, with a head-rest that is semi-cylindrical and made of stone, wood, or metal. 
• Ancient Assyrians, Medes, and Persians had similar beds, and often decorated their furniture with inlays or appliqués of metal, mother-of-pearl, and ivory. 

Tutankhuman's gilded bed from the 14th century BC

Headrest with 2 images of the God Bes, ca. 1539-1190 BCE, Broolyn Museum

• Headrests (in image above) were used to support the head while sleeping. Moreover, they may have been made specifically to support the mummy's head in the coffin, since the offering prayer is inscribed on the supporting column. 
• The oldest account of a bed is likely that of Odysseus: a charpoy woven of rope is featured in the Odyssey. Homer described the inlaying of the woodwork of beds were made of gold, silver, and ivory. The Greek bed had a wooden frame, with a board at the head and bands of hide laced across. At a later period, the bedstead was veneered with expensive wood, occasionally solid ivory veneered with tortoise-shell and silver feet, or bronze. 
• Roman mattresses were stuffed with reeds, hay, or wool, and their feathers were used towards the end of the Roman Republic for luxury. Steps could raise the bedsteads, which contained a board or railing at the back and raised portion of the head. The counterpanes can be Tyrian purple embroidered with golden figures, along with rich hangings falling to the ground that covers the front. 
• Ancient Romans had various kinds of kinds for relaxation purposes such as: 

- Lectus cubicularis = Chamber bed for normal sleeping 

- Lectus Genialis = Marriage bed, typically decorated, placed in the atrium opposite the door. 

- Lectus discubitorius = Table bed for eating normally on their left sides. 3 people to one bed, and the middle place reserved as the most honourable position. 

- Lectus lucubratorius = For studying 

- Lectus funebris / Emortualis = A bed on which the dead are carried to the pyre. 

(b) Post-classical history 

• The ancient Germans laid on the floor on beds of leaves covered with animal skins, or a shallow chest filled with leaves and moss. 
• In the early Middle Ages, carpets were laid on the floor or on a bench against the wall, placed upon the mattresses filled with feathers, wool, or hair, and skins used as covering. Curtains were suspended from the ceiling or from an iron arm protruding from the wall. Historians thought humans back then lain naked in bed, wrapped themselves in large linen sheets that stretched over the cushions. 
• In the 12th century, luxury increased and bedsteads were carved out of wood and decorated with inlaid, carved, and painted ornamentation. Folding beds served as couches by day and contained cushions layered with silk laid upon leather. Whereas at night, a linen sheet was laid out and pillows were laid down, while silk-covered skins served as coverlets. 
• The Carolingian manuscripts illustrated metal bedsteads were significantly higher at the head than at the feet, which continued as this shape until the 13th century in France, with many cushions added to ascend the body to a sloped position. 
• In the 14th century, the woodwork decreased in importance, generally being covered by hangings of rich materials such as silk, velvet or cloth of gold. 
• Later in the 14th century, the Four poster bed (also known as a tester bed) was invented. It was slung from the ceiling or fastened to the walls, which developed later into a room within a room, closed by double curtains. 
• The space between the bed and the wall was called the ruelle, and intimate friends were received there. 
• In the 15th century, beds increased in size, reaching 7 - 8 feet (2.1 - 2.4 m) by 6 - 7 feet (1.8 - 2.1 m). The mattresses were often filled with pea-shucks, straws, or feathers. At the time, superior people carried most of their property about with them, including beds and bed-hangings, and the bedsteads were mostly frameworks that needed covering up. 
• About the beginning of the 16th century, bedsteads were made lighter and more decorative, since the lords hadn't moved out for a long period. 

(c) Modern History 

• The 17th century was labelled the century of magnificent beds, since the style a la duchesse, with tester and curtains only at the head, replaced the more enclosed beds in France. 
• It's known Louis XIV possessed as many as 413 luxurious beds, described in the inventories of his palaces. Some of them contained embroideries enriched with pearls, and figures on a silver or golden ground. 
• In the 18th century, feather pillows were first used as coverings in Germany, which associated with the French bedchamber. 
• The custom of the "bed of justice" was held to symbolise the royal power even more than the throne. It is thought the king of France reclined in this bed during his presence in Parliament, in addition to the princes seated there, the greater officials stood there, and the lesser officials kneeled there. 
• Louis XI was credited with the first use of the bed of justice and the custom lasted until the end of the monarchy. The ceremonial bed was placed in the chambre de parade, and certain people, such as ambassadors or great lords, were honoured and received more intimately than the crowd of courtiers. 
• At Versailles, women received their friends in their beds during periods of mourning, as well as both before and after childbirth, and directly after marriage. 
• During the 17th century, the custom was generalised, probably to avoid the tedious details of etiquette. Until the end of the Ancien Régime, portable beds were used in high society in France, one of which belonged to Charles the Bold. 
• In the 18th century, iron beds began to appear and advertisements proclaimed them as free from insects that infested wooden bedsteads. In England, the closed bed with sliding or folding shutters was common. The four poster was the normal citizen's bed until the middle of the 19th century. 

What are the different bed sizes? 
https://en.wikipedia.org/wiki/Bed_size
Beds come in many different sizes, ranging from infant-sized bassinets and cribs, to small beds for a single person, to large queen and king-size beds for 2 people. Standard bed sizes are based on standard mattress sizes, which vary from country to country, as well as according to the size and degree of ornamentation of the bed frame. The dimensions and names of bed sizes vary between countries worldwide, with most countries using their own standards and terminology. 
The bed you choose to buy and place in your bedroom depends on: 
- The amount of available space in your bedroom. 
- Whether the bed is intended to be used by a single person or shared between 2 (or more) people. 
- Body size and shape, preferred sleeping position and personal preference. 

Beddingstock.com recommended the most suitable bed size for you should be at least 15 cm (6 in) longer than your height if you intend to sleep in it, i.e. 195 cm (76.77 in) long for a 180 cm (70.87 in) tall person. 

The naming standards on different bed sizes and their corresponding actual measurements varies across national standards. 

A. Europe 
In the past, Europe traditionally had more variations in national bed size standards than any region of the world. Bed sizes are defined in centimetres in all European countries, though supplementary imperial equivalents are occasionally displayed in the UK. Today the most common widths sold by pan European retailers are: 
- 90 and 120 cm (35 and 47 in) for single beds. 
- 160, 180 and 200 cm (63, 71 and 79 in) for double beds. 

Other sizes are also available in some European countries, but the widths listed above are the most common in the entire European market. Today, the 200 cm (79 in) length is the most common bed length sold by pan-European retailers. 

Comparison diagram of some of the most common European single and double bed sizes.

(1) France 
In France, single beds are usually 90 cm x 190 cm (35 in x 75 in). 
The most common bed sizes for double beds are: 
- 140 cm x 190 cm (55 in x 75 in)
- 160 cm x 200 cm (63 in x 79 in)
- 180 cm x 200 cm (71 in x 79 in) 

(2) Italy 
In Italy, beds are classified by name and use the term piazza as in "place". Italian standard sizes are: 
i. Una piazza ("one place") or singolo ("single") 
- 80 cm x 190 cm (31 in x 75 in) 
- 90 cm x 190 cm (35 in x 75 in) 

ii. Una piazza e mezza ("1.5 places") 
- 120 cm x 190 cm (47 in x 75 in) 

iii. Piazza e mezza francese ("French 1.5 places") 
- 140 cm x 190 cm (55 in x 75 in) 

iv. Due piazza ("two places") or letto matrimoniale ("matrimonial bed") 
- 160 cm x 190 cm (63 in x 75 in) 
- 180 cm x 190 cm (71 in x 75 in) 

(3) Northern Europe 
These sizes are relevant in Germany, Poland, Belgium, Netherlands, Luxembourg, Norway, Sweden, Denmark, Iceland, Finland, Estonia, Latvia, and Lithuania. 
i. Single 
- 90 cm x 200 cm (35 in x 79 in). 
- Extended variants are usually 210 cm long (83 in) 

ii. Three-quarter (3/4) 
- 140 cm x 200 cm (55 in x 79 in) 

iii. Double 
- 160 cm x 200 cm (63 in x 79 in) 
- 180 cm x 200 cm (71 in x 79 in) = Most common 
- 200 cm x 200 cm (79 in x 79 in) = Common extra-wide bed 

(4) Portugal 
i. Solteiro (single) 
- 80 cm (31 in) 
- 90 cm (35 in) 

ii. Casal (double) 
- 140 cm (55 in) 

iii. Queen size 
- 160 cm (63 in) 

iv. King size 
- 180 cm (71 in) 

v. Super king size 
- 200 cm (79 in) 

(5) Spain 
Standardised lengths are 180, 190 and 200 cm (71, 75 and 79 in). Standardised widths are 80, 90, 105, 120, 135 or 150 (31, 35, 41, 47, 53 or 59 in). The most common bed sizes are: 

i. Individual (single) 
- 90 cm (35 in) 

ii. Matrimonio (marital) 
- 135 or 150 cm (53 or 59 in) 

(6) Switzerland 
i. Single 
- 70 cm x 200 cm (28 in x 79 in) = Only used for double beds with 2 separate mattresses. 
- 80 cm x 200 cm (31 in x 79 in) = Only used for double beds with 2 separate mattresses. 
- 80 cm x 200 cm (35 in x 79 in) = Common single beds 
- 120 cm x 200 cm (47 in x 79 in) = Uncommon, mainly available as European-style futon beds that are sold with a mattress, slats and frame. 
- 140 cm x 200 cm (55 in x 79 in) = Common, especially among young people. 

ii. Double 
- 160 cm x 200 cm (63 in x 79 in) = Common 
- 180 cm x 200 cm (71 in x 79 in) = Common 
- 200 cm x 200 cm (79 in x 79 in) 

(7) UK and Ireland 
Beds are measured according to the size of the mattress they hold, rather than the dimensions of the bed frame itself. Furthermore, British and Irish bed frame sizes aren't standardised and may differ between manufacturers. The table below contains the typical bed sizes from the National Bed Federation (NBF), which is the trade association for the majority of British and Irish bed manufacturers and their suppliers. Most manufacturers use standards defined in feet and inches with the metric indicators not being exact equivalents. According to NBF guidelines, there can be legally be a tolerance of up to + 2 cm (0.8 in) in quoted measurements and the size of the mattress itself. 

Some UK retailers may have a "queen" size referring to one of the above standard sizes, but there is inconsistency regarding which size specifically, with both the small double and super king being prevalent. 

As well as UK standards sizes, common European sizes are also occasionally found in the UK from imports and IKEA, The typical length of an IKEA and other European mattresses is 200 cm (79 in), whereas the UK lengths vary depending on the mattress's width being either 191 or 198 cm (75 or 78 in). 

B. North America 
Mattress sizes use non-numeric labels such as "king" or "full", but are defined in inches. Although most beds were historically "twins" or "doubles", larger mattresses were invented by manufacturers in the mid-1940s, where were later standardised as "queen" and "king" in the 1950s and 60s. Standard mattress depth ranges from the "standard" size of 23 cm (9 in) to "high contour" of up 33 cm (13 in). The table below are the standard ISPA widths and heights in the USA and Canada. 
Note: When dimensions are specified to the half-inch below, the actual dimension called-out by manufacturers or various websites may be + 0.5 inch. 

Proportions of the 7 standard North American bed sizes. 

C. Oceania 
(1) Australia 

Traditional Australian bed sizes

(2) New Zealand 

D. Africa 
(1) South Africa 
South African bed sizes have standard lengths of either 188 or 200 cm (74 or 79 in), with the latter being called XL variants. XL mattresses have the same widths as their non-XL counterparts. The 200 cm (79 in) XL length is recommended for persons over 183 cm (6 ft) tall. 
i. Single 
- 92 cm (36 in) wide 

ii. Three-quarter 
- 107 cm (42 in) wide 

iii. Double 
- 137 cm (54 in) wide 

iv. Queen 
- 152 cm (60 in) wide 

v. King 
- 183 cm (72 in) wide. Equal to the width of 2 mattresses. 

vi. Super King 
- 200 cm (79 in) wide. Available in XL length only. 

E. Asia 
(1) India 
i. Single 
- 91 cm x 198 cm (36 in x 78 in) 

ii. Double 
- 122 cm x 198 cm (48 in x 78 in) 

iii. Queen 
- 152 cm x 198 cm (60 in x 78 in) 

iv. King 
- 183 cm x 198 cm (72 in x 78 in) 

(2) China 
Standard sizes include: 
- Lengths = 190, 195, 200, 210 cm (75, 77, 79, 83 in) 
- Single bed widths = 80, 90, 100, 110, 120 cm (31, 35, 39, 43, 47 in) 
- Double bed widths = 135, 140, 150, 180 cm (53, 55, 59, 71 in) 

Practically, bed sizes are categorised by their widths. The length is typically 200 cm (79 in), but this can vary. 
- 120 cm x 200 cm (47 in x 79 in) 
- 150 cm x 200 cm (59 in x 79 in) 
- 180 cm x 200 cm (71 in x 79 in) 

(3) Indonesia 
The Indonesian standard bed length is always 200 cm (79 in). 
i. Single 
- 90 cm x 200 cm (35 in x 79 in)

ii. Double or twin 
- 120 cm x 200 cm (47 in x 79 in) 

iii. Queen 
- 160 cm x 200 cm (63 in x 79 in) 

iv. King 
- 180 cm x 200 cm (71 in x 79 in) 

v. Super King 
- 200 cm x 200 cm (79 in x 79 in) 

(4) Japan 
Standard Japanese bedding sizes are described in the standard JIS S 1102:2017 Beds for domestic use by the Japanese Standards Association. 

(5) Taiwan 
Traditional Taiwanese bed sizes are locally known as 3 footer, 3.5 footer, 5 footer, 6 footer, and 7 footer, which are Taiwanese small single, twin, full, queen and king respectively. 

Nevertheless, American beds are popular in Taiwan, with bedroom furniture and bedding of U.S. Standard Sizes sold in major Taiwanese cities. 

What are the different types of beds? 

• Adjustable bed = This bed contains a multi-hinged lying surface that can be adjusted to several different positions. Common adjustments include inclining the upper body and elevating the lower body independently. Timby (2008) noted other common features such as heigh adjustment and tilting of the bed to elevate the upper body or the lower body into the Trendelenburg or reverse Trendelenburg positions. These beds are commonly used in hospitals and private homes. 

• Air bed = This bed uses an inflatable mattress, occasionally connected to an electric air pump and adjustable firmness control. The portable version of an air bed can be rolled up and packed for travel or temporary guest use. 

• Bassinet = Also known as a bassinette, or cradle, it is a bed specifically for babies from birth to about 4 months. They are designed to work with fixed legs or casters 

• Box-bed = This bed is enclosed in furniture resembling a cupboard, which originated in western European late medieval furniture. It is closed on all sides by wood panels, hence entering it requires removal of curtains, opening of a door hinge or sliding the doors open. 

• Brass bed = This bed's headboard and footboard is made of brass, and its frames is made of steel.
  
   
• Bunk bed = This bed consists of one bed frame stacked on top of another bed frame, allowing 2 or more beds to occupy the same floor space as 1 bed. They are often used on ships, in the military, in hostels, dormitories, summer camps, prisons, etc. They are normally supported by 4 poles or pillars, with one at each corner of the bed. Reaching the upper bed requires the use of a ladder, which is typically surrounded by a railing to prevent the sleeper from falling out. The US Consumer Product Safety Commission does not recommend the top bunk of a bunk bed to be used for children, due to the safety concerns involving a ladder and the height of the bed. 

• Loft bed = This is an elevated bed similar to a bunk bed minus the lower bed. The free space beneath is used for other furniture, such as a desk, which may be constructed into the loft bed. 

• Camp bed = Also known as a cot, it is a small portable, compact and lightweight bed used in situations where large permanent can't be used. They are generally used armies or organisations, in tourism, and in emergency situations for quick accommodation for victims. They generally consist of a foldable lightweight wood or metal frame, covered with canvas, linen or nylon. 

• Canopy bed = A decorative bed similar to a four-poster bed, featuring posts at each of the 4 corners extending 4 feet high or more above the mattress. Ornate and decorative fabric is regularly draped across the upper space between the posts and a solid swath of cloth creates the ceiling, or canopy directly over the bed. 

• Charpai =  Also known as charpaya, charpoy, khat or manji, it is a traditional woven bed used across South Asia, with regional variations in Afghanistan and Pakistan, North and Central India, Bihar and Myanmar. It is mainly used in warmer regions, hence its net is made out of cotton, natural fibres and date leaves. 

• Cradle (bed) = An infant bed that rocks but doesn't move usually. 

• Curtained bed 

• Daybed = These beds can be used for sleeping, as well as lounging, reclining and seating in common rooms. Their frames Can be made out of wood, metal, or a combination of both. They usually consist of a back and sides and exist in twin size (39 in x 75 in = 99 cm x 191 cm), as well as a trundle to expand sleeping capacity. 

• Futon = A Japanese traditional style of bedding, consisting of a mattress and a duvet. Both elements of a futon bedding set are sufficiently flexible to be aired, folded, and stored away in a large closet during the day. Traditionally, futons are placed on tatami mats, which provide a softer base. They are usually folded away daily and aired in the sun frequently to prevent the development of mold and avoid mites. On the other hand, western-style futons are low, wooden sofa beds, which share the same dimensions as a western bed, but are too thick to fold. They tend to be set up and stored on a slatted frame, which prevents the need to frequently air them. 

• Four Poster bed = This bed has 4 vertical columns, one in each corner, supporting a tester, or upper panel. The tester or panel tend to contain rails for curtains to be pulled around the bed. 

• Hammock = A sling made of fabric, rope, or netting, suspended between 2 or more points, used for swinging, sleeping, or resting. It typically comprises 1 or more cloth panels, or a woven network of twine or thin rope stretched with ropes between 2 firm anchor points such as trees or posts. They were developed by native inhabitants of the Caribbean, Central American, England and South America for the purposes of sleeping. Later they were used aboard ships by sailors to achieve comfort and maximise available space, and by explorers or soldiers travelling in forests. In the 1920s, North American parents used fabric hammocks to contain babies who are at the crawling milestone. Today they are globally popular for relaxation, e.g. as a lightweight bed on camping trips. The word hammock has Spanish roots, originating from a Taíno culture Arawakan word meaning "stretch of cloth" from the Arawak root "maka". 

• Hospital bed = This bed is specifically designed for hospitalised patients or those in need of health care. Its features provide comfort and well-being of the patient and for the convenience of health care workers. Common features include adjustable height for the entire bed, the head, and the feet, adjustable side rails, and electronic buttons to operate both the bed and other nearby electronic devices. Hospital beds are not only found in hospitals, but also in other health care facilities and settings, including nursing homes, assisted living facilities, outpatient clinics, and in home healthcare. 

• Infant bed = Also called a cot in British English, and a crib/cradle in American English, is a small bed designed for infants and young children. They are a historically recent development for the purposes of containing a child capable of standing. 

• Iron bed = These beds' headboards and footboards are made of iron, with the frame rails usually made of steel. They were first built in 17th century Italy to solve the issue of infestation by bed bugs and moths. Today's iron beds are constructed of cold roll, heavy-gauge steel tubing and solid bar stock. 

• Kang bed-stove = It is a traditional long (2 metres or more) platform used for general living, working, entertaining and sleeping. It is often used by residents in the northern part of China, where the climate is freezing during the winter. It is often made of bricks or other forms of fired clay or concrete. Its interior cavity channels the hot exhaust from a firewood / coal fireplace, which leads to a convoluted flue system. The cocking fire from an adjacent room, occasionally from a stove set underground, serves as a kitchen. This maximises the contact time between the exhaust and (indirectly) the interior of the room, thus more heat transfer / recycling back into the room, effectively functioning as a ducted heated system similar to the hypocaust system used by ancient Romans. A kang usually occupies a third to a half of the room's area. 

• Lit a la turque = This bed features 2 scrolling ends and a baldachin, one of many products of the 18th century. 

Drawing of a candle-lit mourning bed (Trauergerüst) for abbess Franziska Christine von Pfalz-Sulzbach, 1776

• Mourning bed = A formal canopied bed, with the deceased, a wax effigy, or symbols of rank. 

• Murphy bed / walled = Also called a wall bed, pull down bed, or fold-down bed, this bed is hinged at 1 end to store vertically against the wall, or inside a closet or cabinet. This bed is named after William Lawrence Murphy (1876-1957), an Irish immigrant in New York who sought a creative method of making better use of apartment space. Although earlier foldup beds already existed, and even available through the Sears, Roebuck & Co. catalog, Murphy introduced pivot and counterbalanced designs that yielded a series of patents, including his June 18, 1912 "Disappearing Bed", and June 27, 1926 "Design for a Bed". Similar to trundle beds, Murphy beds are used for the purposes of saving space, making them the popular option in places with limited living space, such as small houses, apartments, hotels, mobile homes and college dormitories. 

• Ottoman bed = In the UK, it's a type of storage bed in which the storage area is placed underneath the mattress base and accessed by lifting the hinged mattress frame with the assistance of a spring or hydraulic mechanism. 

• Pallet = A thin, lightweight mattress. 

• Petate = A bedroll used in Central America and Mexico, with roots from the Náhuatl word petlatl. It is woven from the fibres of the Palm of petate (Leucothrinax morrisii). 

• Platform bed = Also known as a cabin bed, it is a bed comprising of a raised, level, typically rectangular horizontal solid frame. It often contains rows of flexible wooden slats or latticed structure that supports just a mattress. The platform provides adequate, flexible support and ventilation for a mattress, which eliminates the need for a box-spring or separate bed base (foundation). 

• Polish bed = Also known as a polonaise in English, it is type of canopy bed draped with a baldachin, which originated in 18th-century Poland. The curtain is layered with an elaborate crown-like centrepiece that is connected to the 4 corner posts of the bed frame. 

• Roll-away bed = A bed whom frame folds in half and rolls for easy storage and movement. It is often found in hotels for either free or a nominal fee per night, where more guests than expected may need to sleep in the same room. 

• Rope bed = A pre-modern bed whose wooden frame consists of ropes crossing each other to support the down-filled single mattress. 

• Sleigh bed = A style of bed with curved or scrolled feet and headboards, hence resembles a sled or sleigh. They are often wooden, as a result of the French and American Empire period of the early 19th century. 

• Sofabed = Often called a hide-a-bed, bed-couch, sleeper-sofa, or pullout sofa in the US, it is a sofa or couch that conceals a metal frame and thin mattress underneath its seating cushions, which can be unfolded or opened up to create a bed. 

• State bed = First developed in Early Modern Europe, this bed is from a hierarchy canopy of state. 

• Toddler bed = This small is designed for toddlers, with low side rails (cot sides) on each side to prevent the child from accidentally rolling out of the bed while asleep. It is low to ground to achieve safe and convenient entry and exit for the occupant. 

• Trundle bed = Also known as truckle bed, it is a low, wheeled bed that is stored beneath a normal bed for visitors to use, or as another bed. 

• Vibrating bed = Also known as Magic Fingers bed, it is a coin-operated knick-knack found in a vintage (c. 1960s - early 1980s) motel. For a nominal fee, the mattress uses electronics to vibrate for a period of time. It is used therapeutically to ease back pains or as an erotic aid. 

• Water bed = Also known as a water mattress, or flotation mattress, it is a bed or mattress filled with water. They are used medically to prevent bedsores in invalids. There are hard-sided and soft-sided waterbeds. A hard-sided waterbed consists of a water-consisting mattress inside a rectangular frame of wood resting on a plywood deck sitting on platform. A soft-sided waterbed consists of a water-consisting mattress inside a rectangular frame of sturdy foam, zippered inside a fabric casing, sitting on a platform. There are 3 types of waterbed mattresses: free flow (full wave), semi-waveless, and waveless. Free flow mattresses has no baffles or fibre inserts, whereas the other 2 mattresses do to control water motion and provide back support. 

What are some famous beds? 

• One of the world's largest beds is the Great Bed of Ware, constructed in about 1580. It is 3.26 m (10.7 ft) wide, 3.38 m (11.1 ft) long. It is now displayed in the Victoria and Albert Museum (V&A) in London, along with the Golden Bed invented by William Burges in 1879. 

The Great Bed of Ware

• In 1882, an Indian Maharajah possessed a bed made of solid silver. Each corner of the bed contained a life-sized statue of a naked woman holding a fan. When the Maharajah laid on the bed, his weight triggered a mechanism to drive the women to wave the fan. 
• In 1865, a convertible bed in the form of an upright piano was constructed, which provided home entertainment and saved space in the home. 

What is a bed frame? 

https://en.wikipedia.org/wiki/Bed_frame

A bed frame or bedstead is a component of a bed that is used to position the mattress and base (foundation), as well as serve to support a canopy above. They are usually made of wood or metal, which includes head, foot and side rails, and slats. 

What is a mattress? 

https://en.wikipedia.org/wiki/Mattress

A mattress is a large, rectangular pad that supports a reclining body, which may consist of a quilted or similarly fastened case. This case is typically made of heavy cloth, consisting of materials such as hair, straw, cotton, foam rubber, or a framework of metal springs. They may be filled with air or water. 

When was the mattress invented? 

• The word mattress derives from the Arabic word مَطْرَحٌ (maṭraḥ), defined as "something thrown down" or "place where something is thrown down" and hence "mat, cushion". 
• During the Crusades, Europeans adopted the Arabic method of sleeping on cushions on the floor, and the word materas eventually descended into Middle English through the Romance languages. Wayman (2011) estimated the oldest mattress dates to around 77,000 years ago. 
• Early mattresses contained a variety of natural materials such as straw, feathers, or horse hair. In the first half of the 20th century, a typical mattress sold in North America contained an inner-spring core and cotton batting or fibrefill. 
• Modern mattresses typically contain either an inner spring core or materials such as latex, viscoelastic or other flexible polyurethane foams. Other fill elements include insulator pads over the coils that prevent the bed's upholstery layers from cupping down into the inner spring, as well as polyester fibrefill in the bed's top upholstery layers. 
• In 1899, James Marshall introduced the first individually wrapped pocketed spring coil mattress, commonly known as Marshall coils. 
• The typical mattress sold in North America today is an inner spring, with an interest in all-foam beds and hybrid beds nonetheless. 
• Polyurethane form cores and latex cores are the most popular mattresses sold in Europe. 
• A conventional mattress consists of 2 primary sections - a core or "support layer" and the upholstery or "comfort layer" - enclosed in a thick fabric called the ticking. 
• Upholstery layers contain 3 parts: the insulator, the middle upholstery, and the quilt - which envelop the mattress and provide cushioning and comfort. 

- Air mattress = Also known as an airbed or a blow-up bed, it is an inflatable mattress made of polyvinyl chloride (PVC) or textile-reinforced urethane plastic or rubber. When deflated, the mattress can be compacted and carried around or stored in a small space. Inflating this mattress requires air either being blown into a valve, with a manual or electric pump. 

- Featherbed = A bed composed of a mattress stuffed with feathers. 

- Mattress pad = Also known as a mattress topper, or underpad, it is designed to lie on top of a mattress. They are produced from a variety of materials in order to provide a layer of comfort on top of a worn or uncomfortable mattress. Common types of mattress pads include pillow top, memory foam, latex and electric. 

- Mattress protector = An item of removable bedding that encases a mattress to protect it from dust, body excretions, or liquid spills. They also protect the sleeping person from allergens and irritants such as dust mites, bed bugs, mould, and dead skin (such as dandruff). 

- Memory foam = They contain mainly of polyurethane as well as other chemicals that increase its viscosity and density. It is often known as "viscoelastic" polyurethane foam, or low-resilience polyurethane foam (LRPu). The foam bubbles are open, which generate a matrix for air to move through. Higher-density memory foam softens in response to body heat, which facilitates it to mould to a warm body. 

- Orthopaedic mattress = They are designed to support the joints, back and entire body of people with disorders or deformities of the spine or joints. 

What is a bed base? 

https://en.wikipedia.org/wiki/Bed_base

Also called a foundation, this part of the bed supports the mattress. 
- Box-spring = Also known as divan in some countries, it consists of a sturdy wooden frame layered with cloth and contains springs. 
- Bunkie board = Provides mattress supports for a bunk bed. 

What is a pillow? 

https://en.wikipedia.org/wiki/Pillow

A pillow is a support of the resting body that provides comfort, therapy, or decoration. They can aid sleeping by cushioning the head and neck, or aid the positions of lying down or sitting by cushioning the body.  The word pillow originates from Middle English pilwe, from Old English pyle (akin to Old High German pfuliwi) and from Latin pulvinus. The first known use of the word pillow was before the 12th century. 

When was the pillow first invented?

• The exact origin of the pillow is unknown, but pillow use may have evolved in animals  in prehistoric times. Sanz et al. (2013) noted the earliest examples of pillow use include reptiles and mammals resting their heads on themselves such as their arms or hands, and one another, to support their head and beck. 
• Animals, including humans, evolved to use inanimate objects in their nests such as wood and stone. Since domestication, many animals learned to utilise human-made pillows and cushions, as well as to rest on members of their own and other species. 

• A 2014 National Geographic News article reported tree-dwelling great apes started building sleeping platforms, including wooden pillows, to improve their sleep between 5 and 23 million years ago. 
• Studies found chimpanzees used specifically selected ironwood pillows to achieve 8-9 hours of sleep a night, since such sturdy pillows gave great apes to escape being hunted by night predators and avoid falling out of the trees in its sleep. It's thought this was driven by the evolution of large, energy-consuming brains, leading to longer periods of REM sleep, which subsequently increased their cognitive capacity. 

An ancient Egyptian wooden pillow

• Levy (2002) stated the earliest recorded use of the modern human device dates back to the civilisations of Mesopotamia around 7000 BC, which were only used by the wealthy. In addition, the number of pillows possessed by one exemplified status and affluence. 
• Soane (2007) generalised that pillows were produced worldwide for many years in order to alleviate recurring problems of neck, back, and shoulder pain while sleeping. Aside from comfort, the pillow aided in preventing bugs and insects from entering people's hair, nose, mouth and ears while sleeping. 
• Seath et al. (2009) associated pillow use with the mummies and tombs of Ancient Egypt during the 11th dynasty, dating to 2055-1985 B.C, during which Ancient Egyptian pillows were wooden or stone headrests (image above). These pillows were placed under the heads of the deceased since the human head was regarded as the essence of life and sacred. 
• The Romans and Greeks of ancient Europe grasped the design of the softer type pillow, which were filled with reeds, feathers, and straw in order to achieve softness and comfort. 
• Smith (1975) thought only upper-class people owned these softer pillows, however all classes of people were permitted to use a general type of pillow while sleeping, lying down, or sitting in order to provide support. 
• Fehrman (2013) thought people in ancient Europe began using pillows when attending church to kneel on while praying and playing holy books on. Furthermore, the Romans and Greeks placed pillows beneath the head of the deceased. 

A pottery pillow from the Jīn dynasty 

• Chinese pillows were traditionally solid, though occasionally layered with a softer fabric over them. A 2003 report stated Chinese pillows were composed of a diverse range of materials including bamboo, jade, porcelain, wood, and bronze over many Chinese dynasties, with ceramic pillows being the most popular, nonetheless. 
• The ceramic pillow was first used during the Sui Dynasty between 581 and 618, while mass production occurred during the Tang Dynasty between 618 and 907. These pillows were crafted into different shapes and had pictures of animals, humans, and plants painted on them. 
• A common type of pottery pillow used was Cizhou ware. 
• The production and use of Chinese ceramic pillows peaked during the Song, Jin, and Yuan dynasties between the 10th and 14th century. However, they gradually discontinued during the Ming and Qing dynasties between 1368 and 1911 because of more desirable pillow making materials emerging. 

How are pillows made?

• Pillows are composed of a filler material enclosed in a fabric cover or shell. The cover is usually made of cloth, such as silk, known as the pillow case or pillow slip. Some pillows have a decorative cover called a sham, closed on all sides and contains a slit in the back through which the pillow is positioned. 
• Fillers are picked due to its comfort, resilience, thermal properties, cost, and medical and ethical benefits. The most common synthetic fillers are materials derived from polymer fibres, such as polyester and memory foam. They are cheap and have higher retention of form. 
• Natural fillers include feathers, down, wool, latex, cotton, and buckwheat. More exotic materials include straw, wood, or stone. 
• Down is preferred due to its softness and insulation, but it is expensive due to its relative rarity. 
• Newman (2017) stated down are plucked from live geese, but are already cruelty-free certifications for such products. 
• Traditional Indian pillows are made with kapok, which are fluffy, glossy fruit-fibres of the trees Ceiba pentendra and Bombax ceiba. 

Ceiba pentendra - the fluffy, glossy fruit-fibres of the silk-cotton trees  

How are pillows maintained and managed? 

• The normal lifespan of a pillow is between 2 and 4 years, and manufacturers recommend replacing it after that time for sanitary reasons. 
• It's recommended pillow covers be washed periodically since they physically contact a person's body, especially the head. In addition, they accumulate dust and microbes among the fill. 
• Manufacturers recommend tumble-drying for 15 min weekly to wash them cleanly, and for the heat to kill any dust mites. 

Different types of pillows 
Bed pillow sizes 

- Acupressure pillow = Also known as a spike pillow, nail pillow or fakir pillow, this type of pillow composed of fabrics and plastic nails enclosing a sponge, the latter of which relaxes the user's neck. 

- Bamboo wife = Also known as a Dutch wife, it is a hollow bamboo bolster that was invented and used in East Asian and Southeast Asian countries, including the Philippines, which are regions with hot and humid summers. The open bamboo structure is cool to the touch than fabric pillows or sheets, even in summer heat waves. The user embraces it as if they are a sleeping companion. This sleeping exposes the maximum area of the body to cooling breezes. 

- Bolster = It is a long, narrow pillow or cushion, down or fibre. They are typically firm and for back or arm support or for decorative application. They are not a standard size or shape and regularly have a zipper or hook-and-loop enclosure. Additional support is provided by insertion of a foam into the bolster. The word derives from both Middle and Old English, and is a cognate of the Old English belg, "bag". The first known use of the word "bolster" was before the 12th century. 

- Contour leg pillow = A pillow placed between the legs, typically at knee level, during sleep, which help relieve lower back pain. Rohrlich (2001) found this pillow serves to provide alignment for the pelvis, and to prevent friction between the legs while sleeping. 

- Cushion = A soft bag made from ornamental material, filled with wool, hair, feathers, polyester staple fibre, non-woven material, or torn paper fragments. They can be used for sitting or kneeling upon, or to soften the sturdiness or angular structure of a chair or couch. The word cushion originates from Middle English cushin, from Anglo-French cussin, quissin, from Vulgar Latin coxinus, and from Latin coxa, hip. The first known use of the word cushion was sometime during the 14th century. 

- Dakimakura (抱き枕) = The Japanese words 抱き(daki) and 枕(makura) are defined "to embrace or cling" and "pillow" respectively. Therefore, it is large Japanese pillow with Japanese anime characters printed on them that users cling on to when sleeping. During the late '90s and early 2000s, dakimakura began to merge with otaku culture, which led to the production of pillow covers featuring printed images of bishōjo (美少女) and bishōren (美少年) in lying poses from various anime or bishōjo games. 

- Eye pillow = Also known as a dream pillow, it is a mask-shaped or rectangular punches made from fabric such as cotton or silk and filled with scented or non-scented herbs. They were once used in sickrooms to ease patients' nightmares and to disguise the scent of illness. Herbs such as flax seeds, lavender, chamomile, eucalyptus, and rose petals were used as fillers in eye pillows to help comfort the sick and ease them to sleep. 

- Orthopedic pillow = This pillow is designed to correct body positioning in bed or while lying on any other surface. Its design follows orthopaedic guidelines to achieve the right placement and support of 1 or more specific parts of the body to provide comfortable sleep to the user. They were traditionally made of foam and fibre, but other types currently exist, such as pillows made of memory foam. 

- Sex pillow = It is a specially-designed and typically firm pillow that is used to enhance sexual intercourse. They may contain a high-density urethane core to balance firm support with softness. In addition to the common pillow shapes, there are wedge-shaped, ramp-shaped, prism-shaped, etc. pillows that are suitable for various sexual positions. 

- Speaker pillow = This pillow incorporates loudspeakers, designed as an alternative to headphones connected to portable media players. There are 2 types: one containing a pouch for a smartphone from which audio is played, and the other has a small, flat speaker in the centre of the pillow plugged into an audio device using a headphone jack. 

- Throw pillow = Also known as a toss pillow or a scatter pillow, it is a small decorative soft furnishing item composed of range of textiles including cotton, linen, silk, leather, microfibre, suede, chenille, and velvet. They are commonly used in interior design and exist a range of shapes, sizes and decorative elements such as tassels and piped edges. The most common throw pillow designs are square-shaped and have dimensions of 16 in x 24 in. 

They are loosely placed on sofas or armchairs and also on beds, day beds and floors. They serve both an aesthetic and a functional purpose. In terms of aesthetics, decorative pillows are chosen to combine with colour accents within a room, based on the colours in drapes, walls or area rugs. In terms of function, throw pillows provide back, neck and head support. 

What is a blanket? 

https://en.wikipedia.org/wiki/Blanket

A blanket is a piece of large soft cloth that can either cover up or enfold a substantial portion of the user's body, usually during sleep. It traps radiant bodily heat to help keep the user warm. The term originated from the generalisation of a particular fabric called Blanket fabric, which was an extremely napped woollen weave pioneered by Thomas Blanket, a Flemish weaver who lived in Bristol, England in the 14th century. The earliest use of this term may have derived from the French word for white, blanc. 

There are different types of blankets used for bedding. 

- Afghan = It is a woollen blanket or shawl, typically knitted or crocheted, which is used as a bedspread, or as a decoration on the back of couches or chairs. The word afghan refers to the people of Afghanistan, dating back to 1831, when Thomas Carlyle mentioned it in his Sartor Resartus. The first mention of the word referring to a woven rug was in 1877. The different types of afghan blankets include single-piece, mile-a-minute, join-as-you-go, motif and graphghan. 

- Comforter (American English) = Also known as a doona in Australian English, or a quilt or duvet in British English, it is a type of bedding composed of 2 lengths of fabric or covering sewn together and filled with insulate materials for warmth, traditionally down or feathers, wool or cotton batting, silk, or polyester and other down alternative fibres. 

- Duvet = A type of bedding consisting of a soft flat bag filled with down, feathers, wool, silk or a synthetic alternative. It is usually protected with a removable cover, analogous to a pillow and pillow case. They originated in Europe and were filled with the down feathers of ducks or geese. Back to the days when Vikings existed, duvets of elder down were used by humans on Norway's northern coast. From the 16th century, wealthy people all over Europe began purchasing and using such duvets. 

- Duvet cover = This covers the duvet from protection. They often have a decorative function on the bed, and one may switch the pattern or design for different occasions, or to serve different functions. e.g. A heavier duvet may be used for colder nights. Common duvet covers are made of cotton or a combination of cotton and polyester. 

- Electric blanket = This blanket contains integrated electrical heating wires and a control unit that adjusts the amount of heat it produces. In the UK and Commonwealth countries, the electric underblanket is commonly used, which is placed above the mattress and below the bottom bed sheet. In the USA and Canada, the electric overblanket is commonly used, which is placed above the top bed sheet. 

- Hudson's Bay point blanket = A type of woollen blanket traded by the Hudson's Bay Company (HBC) in British North America (now Canada) and the USA during the 16th and 17th centuries. The company is named after the Hudson Bay and the blankets were traded to First Nations in exchange for beaver pelts. These blankets are sold in Hudson's Bay stores in Canada, and luxury department stores and Hudson's Bay sister chain Lord & Taylor in USA. 

- Patchwork quilt = This quilt comprises of the top layer made of pieces of fabric sewn together to form a unique colourful design. It is composed of 3 layers: the patchwork quilt top, a layer of insulation wedding (batting), and a layer of backing material. These layers are stitched together, either by hand or machine. The quilting either outline the patchwork motifs, or an independent design. Ducey stated quilting's history likely dates back more than 5 millennia and exists in various forms in many cultures. The block-style patchwork quilt is a unique expression of 19th century America, which evolved into a representative folk art. 18th century patchwork was a ladies' leisure activity in both Europe and North America, with the earliest surviving specimens in Wiltshire (1718) and Quebec (1726) composed of silk. 

- Photo blanket = A large rectangular piece of fabric that illustrates images, pictures, or designs, typically with bound edges, used as a blanket or decorative object. Historically, the photo blanket was used as a mean of communication through storytelling, honouring the deceased, and a well-respected form of art. Historians thought photo blankets were historically used by Ancient Egyptians, Mexican Navajo tribes, Nepalese rug makers, tribes in Indonesia and Africa, as well as in Asian cultures. Photo blankets can be woven (afghan or tapestry), knitted or dyed, depending on your preference. 

- Quilt = A multi-layered textile that is traditionally made of 3 layers of fibre: a woven cloth top, a layer of batting or wadding, and a woven back. They are combined using the technique of quilting and the process of sewing. They disclose valuable historical information about their makers by illustrating particular segments of history in tangible, textured ways. In the 21st century, quilts are often presented as non-utilitaran works of art, however were often historically used as bedcovers. 

- Razai = A bedding used in Afghanistan, Iran, Pakistan, North India, Bangladesh and Nepal  that is similar to a duvet or comforter. They typically have a cotton, silk or velvet cover, stuffed with cotton wool. 

- Security blanket = Also known as a comfort object or transitional object, it is an item used to provide psychological comfort, especially in unusual or unique situations, or at bedtime for children. Studies stated security blankets may be induce benefits to a child's or adult's mental and emotional wellbeing, as well as improve sleep quality. 

- Silk comforter (丝棉被, sī mián beì) = A type of bed covering, frequently used as a duvet, also means silk duvet/quilt/blanket in Chinese. This product was originally made and used in late 20th century China, but have become more common in Western market areas. They are popular due to their thermal properties, light weight, and natural hypoallergenic properties. 

- Sleeping bag = An insulated covering that is essentially a lightweight quilt with a zipper to form a tube, acting as a lightweight, portable bedding. It is often used in situations where a person is sleeping outdoors (e.g. when camping, hiking, hill waking, or climbing). It provides warmth and thermal insulation through its synthetic or down insulation. Moreover, it offers protection against wind chill and light precipitation thanks to a water-resistant or water-repellent cover. The bottom of the sleeping bag contains some cushioning, but campers tend to place a sleeping pad or camp cot for additional comfort when sleeping. 
The first records of the modern sleeping bag was called the "Euklisia Rug" (from Ancient Greek, εὖ (well) and κλισία (cot, sleeping-place), patented by mail-order pioneer Pryce Pryce-Jones in 1876. He was a Welsh entrepreneur from Newtown, Montgomeryshire who exported his sleeping bag worldwide in the late 19th century such as Russia, UK and Australia, as well as missionaries in Africa. 

What is a circadian rhythm? 

https://en.wikipedia.org/wiki/Circadian_rhythm

A circadian rhythm is a natural, internal process that regulates the sleep-wave cycle, which repeats approximately every 24 hours. It refers to any biological process exhibiting an endogenous, entrained oscillation of about 24 hours.

The term "circadian" originates from the Latin circa, meaning "around" (or "approximately"), and diēm, meaning "day".

How was concept of the circadian rhythm coined?

• Around the 4th century BC, a ship captain named Androsthenes, who served under Alexander the Great, first described diurnal leaf movements of a tamarind tree. 
• The observation of a circadian or diurnal process in human was mentioned in Chinese medical texts dated to approximately the 13th century, including the Noon and Midnight Manual, the Mnemonic Rhyme to Aid in the Selection of Acu-points According to the Diurnal Cycle, and the Day of the Month and the Season of the Year. 
• In 1729, French scientist Jean-Jacques d'Ortous de Mairan recorded the first observation of an endogenous circadian oscillation. He observed the continuous 24-hour patterns in the movement of the leaves of the plant Mimosa pudica even when the plants were kept in constant darkness (i.e. no sunlight). 
• In 1896, Patrick and Gilbert observed sleepiness increases and decreases with a period of approximately 24 hours during a prolonged period of sleep deprivation. 
• In 1918, J.S. Szymanski demonstrated that animals are capable of maintaining 24-hour activity patterns in the absence of external cues such as light and changes in temperature. 
• In the 20th century, Auguste Forel, Ingeborg Beling, and Oskar Wahl performed extensive experiments to investigate whether the cause of the rhythmic feeding times of bees is an endogenous clock. 
• In 1935, 2 German zoologists, Hans Kalmus & Erwin Bünning, independently discovered the circadian rhythm in the fruit fly Drosophila melanogaster. 
• In 1954, Colin Pittendrigh reported eclosion (i.e. the process of pupa turning into an adult) in D. pseudoobscura was a circadian behaviour. He demonstrated that the period of eclosion was delayed but not halted when the temperature was decreased, which indicated an internal biological clock controlled circadian rhythm. 
• The term circadian was coined by Franz Halberg in 1959, derived from circa (about) and dies (day). His definition was "it was serve to imply that certain physiological periods are close to 24 hours, if not exactly that length. Herein, circadian might be applied to all "24-hour" rhythms, whether or not their periods, individually or on the average, are different from 24 hours, longer or shorter, by a few minutes or hours." 
• In 1977, the International Committee on Nomenclature of the International Society for Chronobiology formally adopted the definition of "circadian", stating: "Relating to biologic variations or rhythms with a frequency of 1 cycle in 24 + 4 hours, circa (about, approximately) and dies (day or 24 hours). Note: term describes rhythms with an about 24-hour cycle length, whether they are frequency-synchronised with (acceptable) or are desynchronised or free-running from the local environmental time scale, with periods of slightly yet consistently different from 24-hours." 
• In 1971, Ron Konopka and Seymour Benzer identified the first clock mutant in Drosophila and labelled it "period" (per) gene, which was also the first discovered genetic determinant of behavioural rhythmicity. 
• In 1984, the per gene was isolated by 2 teams of scientists; (1) Konopka, Jeffrey Hall, Michael Roshbash and co. demonstrated that the per locus is centre of the circadian rhythm, and that loss of per ceases circadian rhythm. 
• Simultaneously, Michael W. Young's team reported similar findings of per, and discovered the gene covers 7.1-kb interval on the X-chromosone and encodes a 4.5-kb poly(A)+ RNA. They continued to discover the key genes and neurons in Drosophila circadian system, for which Hall, Rosbash and Young received the Nobel Prize in Physiology or Medicine 2017. 
• In 1994, Joseph Takahashi discovered the first mammalian circadian clock mutation (clockΔ19) in mice. Nevertheless, Debruyne et al. (2006) found deletion of clock gene doesn't lead to a behavioural phenotype (i.e. normal circadian rhythms), which queries its importance in rhythm generation. 
• In 2001, an extended Utah family by Chris Jones discovered the first human clock mutation, which was genetically identified by Ying-Hui Fu and Louis Ptacek. Affected individuals become 'morning larks' with 4 hour advanced sleep and other rhythms. This form of Familial Advanced Sleep Phase is caused by a single amino acid substitution, S662->G, in the human PER2 protein. 

What is the criteria of a circadian rhythm?
For a biological rhythm to be circadian, it must satisfy 3 general criteria:

1. The rhythm has an endogenous free-running period that lasts approximately 24 hours.

• The rhythm persists in constant conditions, (i.e. constant darkness) with a period of about 24 hours. 
• The period of the rhythm in constant conditions is called the 'free-running period', denoted by tau (τ). 
• The rationale for this criterion is to distinguish circadian rhythms from simple resources to daily external cues. 
• A rhythm can't be determined as endogenous unless it has been tested and demonstrated to persist in conditions without external periodic input. 
• In diurnal animals, τ is slightly longer than 24 hours, and vice versa in nocturnal animals.

2. The rhythms are entrainable. 

• When the rhythm can be reset by exposure to external stimuli (such as light and heat), it is entrainable (process is called entrainment). 
• The external stimulus used to entrain a rhythm is called the Zeitgeber, or "time giver". 
• E.g. When humans travel across time zones, their biological clock adjust to the local time. They will usually experience jet lag before entrainment of their circadian clock syncs it with local time. 

3. The rhythms exhibit temperature compensation. 

• This means the rhythms can maintain circadian periodicity over a range of physiological temperatures. 
• Many organisms live at a diverse range of temperatures, and variations in thermal energy affects the kinetics of all molecular processes in their cell(s). 
• To keep track of time, the organism's circadian clock must maintain approximately a 24-hour periodicity in spite of changing kinetics, a property known as 'temperature compensation'. 
• The Q10 Temperature Coefficient measures this temperature compensation effect. If the Q10 coefficient remains approximately 1 as temperature increases, the rhythm is determined to be temperature-compensated. 

How did the circadian rhythm evolve? 

• It's known circadian rhythms give organisms the capability to anticipate and prepare for pinpoint and consistent environmental changes, allowing them to take advantage of environmental resources (e.g. light and food). Researchers suggested circadian rhythms put organisms at a selective advantage in evolutionary contexts. 
• Sharma (2003) found rhythmicity plays a role in regulation and coordination of internal metabolic processes, e.g. coordination of the environment. 
• The factors driving the evolution of circadian rhythms is still unknown and many hypotheses have been proposed to answer this question. 
• Edgar et al. (2012) underscored the importance of co-evolution of redox proteins with circadian oscillators in all 3 domains of life following the Great Oxidation Event approximately 2.3 billion years ago. 
• The popular hypothesis is circadian changes in environmental oxygen levels and the production of reactive oxygen species (ROS) in the presence of daylight likely drive the evolution of circadian rhythms to anticipate, and therefore nullify, damaging redox reactions on a daily basis. 
• The simplest known circadian clocks are bacterial circadian rhythms, demonstrated by the prokaryote cyanobacteria. Hut & Beersma (2011) found the circadian clock of Synechococcus elongatus reconstitutes in vitro with 3 proteins of their central oscillator, which are KaiA, KaiB & KaiC. This clock demonstrated sustainability for 22 hours over several days upon addition of ATP. 
• Researchers identified a defect in the human homologue of the Drosophila "period" gene as the cause of the sleep disorder FASPS (Familial advanced sleep phase syndrome). This finding highlighted the conserved nature of the molecular circadian clock through evolution. 

How important is the circadian rhythm in animals? 

• Circadian rhythmicity exists in the sleeping and feeding patterns of animals, including humans, such as core body temperature, brain wave activity, hormone production, cell regeneration and photoperiodism. 
• Timely predictions of seasonal periods of weather conditions, food availability, or predator activity may have been critical for the survival of many species. 

I. Effect of circadian disruption 

• Delezie et al. (2012) found the deletion of the Rev-ErbA alpha clock gene promotes diet-induced obesity and alters the balance between glucose and lipid metabolism predisposing to diabetes in mice. 

II. Arctic animals 

• Norwegian researchers at the University of Tromsø demonstrated that particular Arctic animals (ptarmigan, reindeer) exhibited circadian rhythms only during times with daily sunrises and sunsets. e.g. Reindeer on Svalberd at 78 degrees North demonstrated such rhythms only in local autumn and spring. 
• Folk et al. (2006) discovered that day-living ground squirrels and nocturnal porcupines strictly maintain their respective circadian rhythms through 82 days and nights. They conjectured that these 2 rodent species judge the apparent distance between the sun and the horizon is the shortest once a day, and therefore has a prominent signal to entrain by. 

IV. Flying insects 

• A 2005 study found the circadian rhythm controlled mating behaviour in certain moth species such as Spodoptera littoralis, where females produce specific pheromones that attracts and resets the male circadian rhythm to bring about mating at night. 

How does circadian rhythm work in plants? 

http://www.plantcell.org/content/18/4/792

https://plantcellbiology.masters.grkraj.org/html/Plant_Growth_And_Development14-Circadian_Rhythm_And_Biological_Clock.htm

https://www.semanticscholar.org/paper/All-in-good-time%3A-the-Arabidopsis-circadian-clock.-Barak-Tobin/631a31d32f62bf182ac1dd4ff81dcde95b0ef723/figure/0

Plant clocks control a plethora of biological processes.

Source: All in good time: the Arabidopsis circadian clock, Barak et al., 2000, doi: 10.1016/S1360-1385(00)01785-4

• Circadian rhythms inform plants the current season and the best chance of attracting pollinators. Webb (2003) found examples of behaviours demonstrating such rhythms include leaf movement, growth, germination, stomatal / gas exchange, enzyme activity, photosynthetic activity, and fragrance emission, etc.. 
• Circadian rhythms occur as a plant entrains to synchronise with the light cycle of its surrounding environment. They are endogenously generated and self-sustaining that are relatively consistent over a range of ambient temperatures. 
• Features of plant circadian rhythms include 2 interacting transcription-translation feedback loops: proteins containing PAS domains that facilitate protein-protein interactions, and several photoreceptors that adjust the clock to different light conditions. 
• McClung (2006) suggested that when plants anticipate environmental changes, they initiate appropriate changes in its physiological state, which provide an evolutionary advantage. 
• Plants have a wide variety of photoreceptors to sense changing light conditions in order to synchronise their internal clocks to the environment. 
• Red & blue light are absorbed by several phytochromes and crytochromes. 
• PhyA is the main phytochrome in seedlings that grow in the dark but rapidly degrade in light to generate Cry1. 
• Phytochromes B-E are more stable in combination with phyB, the main phytochrome in seedlings grown in the light. 
• The cryptochrome (cry) gene is a light-sensitive component of the circadian clock and is postulated to be involved both as a photoreceptor and as part of the clock's endogenous pacemaker mechanism. 
• Webb (2003) explained crytochromes 1-2 (involved in blue-UVA) maintain the period length in the clock through a whole range of light conditions. 

A Molecular Model of the Arabidopsis thaliana Circadian Oscillator.

- Genes are indicated by solid boxes with the gene names indicated at the left.
- Proteins are indicated by oval and oblong shapes, with the protein name indicated within the shape.
- Transcription and translation are indicated by dashed lines.
- Protein activity is indicated with solid lines, with lines ending in arrowheads indicating positive action and lines ending in perpendicular dashes indicating negative action.
- The core CCA1/LHY/TOC1 feedback loop is highlighted in green with thick lines and closed shapes.
- Phosphorylation of LHY and CCA1 by CK2 is indicated with circled Ps.
- Shaded area indicates activities peaking in the subjective night, and the white area indicates activates peaking during the subjective day.

Source: Plant Circadian Rhythms, McClung, 2006, doi: http://doi.org/10.1105/tpc.106.040980

• The central oscillator generates a self-sustaining rhythm and is compelled by 2 interacting feedbacks that are active at different times of the day.  
• The morning loop includes the CCA1 (Circadian and Clock-Associated 1) and LHY (Late Elongated Hypocotyl), which encode closely related MYB transcription factors that regulate circadian rhythms in Arabidopsis, and PRR 7 and 9 (Pseudo-Response Regulators). 
• The evening loop includes GI (Gigantea) and ELF4, both of which are involved in regulation of flowering time genes. 
• When CCA1 and CHY are overexpressed (under constant light or dark conditions), plants become arrhythmic, and mRNA signals decrease, which leads to a negative feedback loop. 
• Gene expression of CCA1 and LHY oscillates and peaks in the early morning, whereas TOC1 gene expression oscillates and peaks in the early evening. 
• McClung (2006) hypothesised that these 3 genes create a negative feedback loop model that illustrates overexpression of CCA1 and LHY repressing TOC1 and overexpression of TOC1 acting as a positive regulator of CCA1 and LHY. 
• Millar et al. (2012) demonstrated that TOC1 represses CCA1, LHY and PRR7,9 in the morning loop, as well as GI and ELF4 in the evening loop. 
• Pokhilko et al. (2012) used computational modelling of TOC1 gene functions and interactions to imply a modification of the plant circadian clock as a triple negative-component repressilator model rather than the positive/negative-element feedback loop characterising the clock in mammals. 
• Ma et al. (2018) discovered that expression of PRR5 and TOC1 hnRNA nascent transcripts follow the same oscillatory pattern as processed mRNA transcripts rhythmically in A.thaliana. 
• LNKs binds to the 5' region of PRR5 and TOC1 and interacts with RNAP II and other transcription factors. Furthermore, RVE8-LNKs interaction allows a histone-methylation pattern (H3K4me3) to be modified and the histone-modification itself resembles the oscillation of clock gene expression. 
• Dodd et al. (2005) discovered that matching a plant's circadian rhythm to its external environment's light and dark cycles positive impacts the A. thaliana overall, such as desirably synchronising its processes and controlling chlorophyll levels relative to sunlight levels. 
• Dodd et al. (2015) proposed the metabolic dawn hypothesis that suggested sugars created by photosynthesis help regulate the circadian rhythm and certain photosynthetic and metabolic pathways. 
• Haydon et al. (2013) found the sugars produced by photosynthesis in high light levels repress PRR7, which increase the expression of CCA1. On the other hand , decreased photosynthetic sugar levels increase PRR7 expression and decrease CCA1 expression. Researchers described this feedback loop between CCA1 and PRR7 as the cause of 'metabolic dawn'. 

Describe Drosophila circadian rhythm 

https://en.wikipedia.org/wiki/Drosophila_circadian_rhythm

Key centres of the mammalian and Drosophila brains (A) and the circadian system in Drosophila (B)

• In Drosophila, there are 2 distinct groups of circadian groups, namely the clock neurons and the clock genes. Both act collectively to create the 24-hour cycle of rest and activity. 
• Veleri & Wülbeck (2004) found the compound eyes, ocelli and Hofbauer-Buchner eyelets (HB eyelets) are the direct external photoreceptor organs that perceive light sources, which can function in complete darkness nonetheless. 
• Rieger et al. (2003) found the drosophila's compound eyes help it distinguish long days from constant light, and the normal masking effects of light, such as inducing activity by light and inhibition by darkness. 
• Yoshii et al. (2012) identified 2 distinct activity peaks denoted M (morning) peaks, and E (evening) peaks, both of which monitor the different day lengths in different seasons of the year. 
• Schlichting et al. (2014) explained that rhodopsins 1 and 6 (light-sensitive proteins in the eye) are essential for activation of the M and E oscillations. 
• Nitabach & Taghert (2008) estimated about 150 neurons are in the brain regulate the circadian rhythm upon detection of environmental light. These clock neurons are located in distinct clusters in the central brain. 
• Yoshii et al. (2015) discovered the large and small lateral ventral neurons (l-LNvs and s-LNvs) of the optic lobe, which produce pigment dispersing factor (PDF), a neuropeptide that functions as a circadian neuromodulator between different clock neurons. 

Molecular interactions of clock genes and proteins during Drosophila circadian rhythm.

• Drosophila circadian rhythms keep track of time through daily variations of clock-related proteins that interact in transcription-translation feedback loop. Boothroyd & Young (2008) found the core clock mechanism comprises of 2 interdependent feedback loops, which are the PER/TIM loop and the CLK/CYC loop. 
• During the day, the CLK/CYC loop activates, producing both Clock protein and cycle protein. The CLK/CYC heterodimer act as transcription factors and combine to initiate the transcription of the per and tim genes by attaching to a promoter element called E-box, around mid-day. 
• DNA is then transcribed to create PER mRNA and TIM mRNA. Subsequently, PER and TIM proteins are synthesised in the cytoplasm over the course of the day. Nitabach & Taghert (2008) evaluated their mRNA levels peak early in the evening and protein levels peak around dawn. 
• However the PER and TIM protein levels are kept constant at low levels until dusk, since the doubletime (dbt) gene is also activated during daylight. 
• DBT protein triggers post-translational modifications, including phosphorylation and turnover of monomeric PER proteins. As PER is translated in the cytoplasm, DBT (casein kinase 1ε) and casein kinase 2ε (synthesised by And and Tik) actively phosphorylates it as a precursor to premature degradation. 
• Grima et al. (2002) noted the actual degradation process occurs via the ubiquitin-proteasome pathway, performed by a ubiquitin ligase called Slimb (supernumerary limbs). Simultaneously, TIM is itself phosphorylated by shaggy, whose activity decreases after sunset. 
• DBT gradually depletes, and elimination of DBT facilities stabilisation of PER by physical association with TIM, thereby PER and TIM are maximally produced at dusk. Simultaneously, CLK/CYC also directly activates vri and Pdp1 (PAR domain protein 1). 
• 3-6 hours earlier, VRI initially aggregates, and begins to repress Clk, however PDP1 competes by activating Clk. PER/TIM dimer assembles in the early evening and translocates in a coordinated manner into the nucleus hours later, and binds to CLK/CYC dimers. Helfrich-Förster (2005) observed bound PER completely inhibits the transcriptional activity of CLK and CYC. 
• In the early morning, light causes the degradation of PER and TIM proteins in a network of transcriptional activation and repression. Firstly, light activates the cry gene in the clock neurons. Although CRY is produced inside the brain, it is sensitive to UV and blue light, demonstrating efficiency in signalling the neurons the onset of light. Moreover, CRY irreversibly and directly binds to TIM, triggering its breakdown  through proteasome-dependent ubiquitin-mediated degradation. 
• Busza et al. (2004) found CRY's photolyase homology domain's function is detection of light and phototransduction, whereas the carboxyl-terminal domain's function is regulation of CRY stability, CRY-TIM interaction, and circadian photosensitivity. 
• Koh, Zheng & Sehgal (2006) stated a different protein JET promotes the ubiquitination and subsequent degradation. Therefore, PER/TIM dimer segregates, and the unbound PER is destabilised. PER subsequently undergoes progressive phosphorylation and ultimately degradation. 
• Lalchhandama (2017) found the lack of PER and TIM activates clk and cyc genes, allowing the circadian clock to reset for the commencement of the next circadian cycle. 

How does circadian rhythm work in mammals? 

https://en.wikipedia.org/wiki/Light_effects_on_circadian_rhythm

A variation of an eskinogram that illustrates how light and darkness impacts circadian rhythms and related physiology and behaviour through the Suprachiasmatic Nucleus (SCN) in humans.

• The primary circadian clock in mammals is located in the suprachiasmatic nucleus (SCN), which are a pair of distinct group of neurons in the hypothalamus. When the SCN is lesioned, the regular circadian rhythm disappears. 
• The retina of the eye contains photoreceptors (cones and rods) and photosensitive ganglion cells that collect information about illumination through the eyes. The ganglion cells then directly project to the SCN, where they assist in entraining (synchronising) the master circadian clock. 
• Specialised ganglion cells contain the photopigment melanopsin and their signals travel along the retinohypothalamic tract to the SCN. A 2010 study found removal and culturing of the SCN cells lead to maintenance of their own rhythm without any external cues. 
• The SCN gathers information on the lengths of the day and night from the retina and processes it before transmitting it to the pineal gland, a small pine-cone shaped brain structure in the epithalamus. The pineal gland subsequently releases a sleep-inducing hormone called melatonin. Secreted melatonin levels is the highest at night and recedes during the day, which help provide the brain information about the length of night. 
• Kalpesh (2016) implied that pineal melatonin provides feedback on SCN rhythmicity to regulate circadian patterns of activity and other processes. However, more research is required to understand the nature and system-level significance of this feedback. 

I. Humans 

When eyes receive light from the sun, the pineal gland's production of melatonin is inhibited and the hormones produced keep the human awake. When the eyes don't receive much light, melatonin is produced in the pineal gland and the human feels tired. 

• Cromie (1999) estimated the natural human rhythm to be closer to 24 hours and 11 minutes, close to the solar day. 
• Duffy et al. (2011) observed sex differences with the circadian period for women being shorter (24.09 hours) than for men (24.19 hours). They found women tended to woke up earlier than men and demonstrated preference for morning activities than men. However the biological mechanisms behind their differences warrant further investigation. 

II. Biological markers and effects 

The classic phase markers for measuring the timing of a mammal's circadian rhythm are: 

- Melatonin secretion by the pineal gland

- Minimum core body temperature 

- Plasma level of cortisol 

• Although variation occurs among normal chronotypes, the average human adult's temperature reaches its minimum at about 5:00 am, about 2 hours before habitual wake time. 
• Baehr et al. (2000) discovered that the daily body temperature minimum in young adults occurred at around 4:00 am for morning types and around 6:00 am for evening types. 
• During daytime, melatonin levels are relatively low from the body system. The dim light melatonin onset (DLMO) occurring at around 9:00 pm is measurable in the blood or saliva, and its metabolite can be detected in urine during the morning. 
• Benloucif et al. (2005) observed that melatonin phase markers demonstrated stability and reliability in regards to timing of sleep. Moreover, the researchers discovered that both sleep offset and melatonin offset correlated with phase markers. Another reliable and stable marker is the declining phase of the melatonin levels. 

Other physiological changes that transpire according to the circadian rhythm include heart rate, cellular processes such as:

- Oxidative stress

- Cell metabolism

- Immune and inflammatory responses

- Epigenetic modification

- Hypoxia / hyperoxia

- Response pathways

- Endoplasmic reticular stress 

- Autophagy 

- Regulation of the stem cell environment. 

• Degaute et al. (1991) observed young men's heart rate approaches its lowest average rate during sleep, and its highest average rate shortly after waking. 
• Quartel (2014) learned that body temperature had no effect on performance on psychological tests. A possible reason for this finding include the evolutionary pressure for higher cognitive function compared to the other areas of function. 

III. Outside the "master clock" 

• Mohawk et al. (2013) theorised "almost every cell in the body contains a circadian clock" that operate independently of each other. 
• Recent studies found these "peripheral oscillators" in the adrenal gland, oesophagus, lungs, liver, pancreas, spleen, thymus and skin, as well as the olfactory bulb and prostate. However more research is required to understand this phenomenon. 

a. Process S 

• Process S is defined as the driver of sleep in an organism that has been awake for a considerable period of time. Schwartz et al. (2017) explained that process S is prompted by the depletion of Glycogen and accumulation of Adenosine in the forebrain that disinhibits the ventrolateral preoptic nucleus, as well as facilitates the inhibition of the ascending reticular activating system. 

I'll delve into the effects of sleep deprivation and sleep debt later. 

b. Distribution 

• Polyphasic sleep consists of more than 1 sleep sessions in a 24-hour cycle, while monophasic sleep consists of only 1 sleep session. Dijk & Edgar (1999) found humans tended to switch between sleep and wakefulness frequently (i.e. exhibit more polyphasic sleep) when they experienced boredom. 
• Wehr (1999) found humans tended to experience bimodal sleep inside a dark room for 14 hours under experimental conditions, with 2 sleep periods occurring at the beginning and end of the session. 
• Different characteristic sleep patterns such as "early bird" and "night owl" are known as chronotypes, which can be influenced by genetics, sex, age and lifestyle habits. 

c. Naps 

• Whenever you are on a long road trip, completing a school assignment, or creating spreadsheet documents, you tend to feel sleepy and proceed to take naps. A nap is defined as a short period of sleep, usually during daytime hours. Researchers have been investigating the psychological and physical effects of naps on humans, including the 30-minute nap or 15-minute power nap. 
• In Spanish, an early afternoon nap, often after the midday meal, is called a siesta, which is a common tradition in countries throughout the Mediterranean, Southern Europe and Mainland China such as Spain, Philippines, and many Hispanic American countries. Siesta derives from the Latin word hora septa meaning "sixth hour" (counting from dawn, hence "midday rest"). 
• Naska et al. (2007) found the siesta habit decreased mortality by coronary diseases by 37%, due to decreased cardiovascular stress mediated by daytime sleep. Tanaka & Tamura (2015) found short naps at mid-day and mild evening exercises improved sleep, performance in cognitive tasks, and mental health in elderly people. 

d. Genetics of sleep 

• Jones et al. (2016) listed a number of genes that may play a role in sleep, including ABCC9, DEC2, Dopamine receptor D2, and variants near PAX8 and VRK2. 

e. Quality of sleep 

• Measuring the quality of sleep currently rely on objective and subjective scales, as there is no unit of measurement for it. 
• 'Objective sleep' is defined as the level of difficulty falling asleep and remaining in a sleep state, and the number of times waking up during a single night. 
• 'Subjective sleep' is defined as the sense of being rested and regenerated after waking up from sleep. 
• Harvey et al. (2002) found insomniacs demonstrated higher demand in their evaluations of sleep quality compared to subjects with no sleep problems. 

Why do we sleep? 

https://en.wikipedia.org/wiki/Sleep

Sleep is defined as a condition of body and mind which typically recurs for hours every night, in which the: 
- Nervous system is inactive
- Eyes closed
- Postural muscles relaxed and,
- Consciousness practically suspended. 

In scientific terms, it is a naturally recurring state of mind and body, typified by: 
- Altered consciousness
- Decreased muscle activity and inhibition of nearly all voluntary muscles during rapid eye movement (REM) sleep and, 
- Decreased interactions with surroundings. 

As you sleep, a number of prominent physiological processes occur in your brain. Your brain uses considerably less compared to awakened state, i.e. basal metabolic rate. In certain brain regions, neural activity decreases, allowing the replenishment of the brain's supply of Adenosine Triphosphate (ATP), the molecule used for short-term storage and transport of energy. Siegel (2008) estimated the body's overall energy consumption decreases by roughly 20% in quiet waking state due to the brain. When you're asleep, you perceive fewer stimuli, but could potentially respond to loud noises and other salient sensory events such as your phone alarm, fire alarm, smelling salts, screaming, and creaking noises. 

Discuss the neuroscience of sleep 

https://en.wikipedia.org/wiki/Neuroscience_of_sleep

An artist's illustration depicting REM sleep.

Sleep is a popular topic of study in neuroscience, physiology and psychology. Advances in technology (e.g. improved imaging techniques) and proliferation of neuroscience research since the 1950s has made the study of sleep more prominent in neuroscience. The fundamental questions in the neuroscientific study of sleep include: 

1. What are the correlates of sleep i.e. what are the minimal set of events that could confirm that the organism is sleeping rather than pretending to sleep?
2. How is sleep triggered and regulated by the brain and the nervous system? 
3. What happens in the brain during sleep? 
4. How can we understand sleep function based on physiological changes in the brain? 
5. What causes various sleep disorders and how can they be treated? 

Other areas of modern neuroscience sleep research include the evolution of sleep, sleep during development and ageing, animal sleep, mechanism of effects of drugs, dreams and nightmares, and stages of arousal between sleep and wakefulness. 

A 2000 study categorised sleep into 2 broad types: Non-Rapid Eye Movement (NREM) sleep or Rapid Eye Movement (REM) sleep. Moreover, it represented waking as the 3 major modes of consciousness, neural activity, and physiological regulation. NREM sleep itself is divided into multiple steps - N1, N2 and N3. Each sleep session proceeds in 90-minute cycles of REM and NREM, following the order of N1 -> N2 -> N3 -> REM. As you fall asleep, the body parameters that decrease include metabolic rate, body temperature, heart rate, breathing rate, and energy expenditure. On the other hand, brain waves are found to slow down and amplify. Though all reflexes remain active, less of the excitatory neurotransmitter acetylcholine is available in the brain during sleep. In order to maintain a thermally friendly environment, you tend to curl up into a ball when feeling chilly. In 1975-77, Hobson and McCarley proposed the activation-synthesis hypothesis in order to explain the alternation between REM and non-REM stages in terms of cycling, and reciprocally influential neurotransmitter systems. 

What are the correlates of sleep? 

• Symptomatically, sleep is characterised by a lack of reactivity to sensory inputs, low motor outputs, reduced conscious awareness, and rapid reversibility to wakefulness. However, it is difficult to translate these characteristics to a biological definition since no single neural pathway is responsible for the generation and regulation of sleep. Jones (2009) found the thalamus is only deactivated in terms of transmission of sensory information to the cerebral cortex. 
• Tononi & Cirelli (2006) observed sympathetic nerve activity decreases and parasympathetic nerve activity increases in non-REM sleep, and both blood pressure and heart rate increases and both homeostatic response and muscle tone decreases in REM sleep. 
• In recent times, the modern biological definitions of sleep is based on overall brain activity in the form of characteristic EEG patterns. Dement & Kleitman (1957) found each stage of sleep and wakefulness elicits a characteristic EEG pattern, which will be discussed later. 

What is going on inside the brain during sleep? 

Understanding the functions of sleep requires thorough investigation of the electrical activity in different regions of the brain. Contrary to popular belief, the brain still exerts mental activity during all stages of sleep, with sleep intensity of a particular region being homeostatically associated with the corresponding amount of activity prior to the sleep stage. Sleep scientists use imaging techniques such as PET and fMRI, as well as EEG (electroencephalography), to identify specific regions involved in generating the characteristic wave signals and understand their possible functions. 

(a) Historical development of the stages model 

• In 1937, Alfred Lee Loomis and his colleagues first described the stages of sleep by separating the different EEG features of sleep into 5 levels (A, B, C, D & E), and representing the spectrum from wakefulness to deep sleep. 
• In 1953, REM sleep was discovered as its distinct wake pattern, and hence William C. Dement and Nathaniel Kleitman reclassified sleep into 4 NREM stages and REM. 
• In 1968, the staging criteria were standardised by Allan Rechtschaffen and Anthony Kales in the "R&K sleep scoring manual". 
• In the R&K standard, NREM sleep was divided into 4 stages, with the slow-wave sleep comprised of stages 3 and 4. In stage 3, delta (δ) waves comprised of less than 50% of the total wave patterns, but more than 50% in stage 4. 
• After the 2004 review of the R&K scoring system commissioned by the AASM Visual Scoring Task Force, stages 3 and 4 were combined into Stage N3, and additional parameters were included such as arousal, respiratory, cardiac and movement events. 

(b) NREM sleep activity 
https://en.wikipedia.org/wiki/Non-rapid_eye_movement_sleep

• Constituting about 80% of all sleep in adult humans, NREM sleep is characterised by reduced global and regional cerebral blood flow. Brain regions known to deactivated in the NREM stage of sleep include precuneus, basal forebrain, basal ganglia and areas of the cortex. Studies conducted in 1997 found the ventromedial prefrontal cortex is least active while the primary cortex is the least deactivated. 
• NREM is characterised by slow oscillations, spindles, and delta waves. McGinty & Sterman (1968) demonstrated slow oscillations are generated by the cortex, as lesions in regions other than cortex didn't affect them. 
• Reciprocally connected thalamic and cortical neural circuits are known to generate delta waves. Although the thalamus halts relaying sensory information to the brain during sleep, it continues to transmit signals to its cortical projections. Hutt (2011) found these waves are produced by the thalamus independent of the cortex, while the cortex is responsible for simultaneous firing of large bundles of neurons. 
• The pacemaker of the sleep spindles is thought to be the thalamic reticular nucleus, since rhythmic stimulation of the thalamus leads to increased secondary depolarisation in the cortical neurons, which further increased the amplitude of firing, triggering self-sustained activity. Steriade (1997) implied the sleep spindles disconnect the cortex from sensory input and allow entry of calcium ions into cells, thus playing an important role in neuroplasticity. 
• Polysomnography (PSG) is a test used in the study of sleep, with the test result called a polysomnogram. The images in each NREM stage represent 30-second epochs (30 seconds of data), which includes data from both eyes, EEG, chin, microphone, EKG, legs, nasal/oral air flow, thermistor, thoracic effort, abdominal effort, oximetry, and body position, in that order. The EEG is highlighted by a red box, with sleep spindles in the stage 2 figure underlined in red. 

1. NREM1 (Stage 1: N1 - Light sleep, somnolence, drowsy sleep - 5-10% of total sleep in adults) 

This stage of sleep usually occurs between sleep and wakefulness, and occasionally occurs between periods of deeper sleep and periods of REM sleep. Muscles are still active, and the eyes roll slowly, opening and closing to a certain extent. The brain transitions from alpha (α) waves having a frequency range of 8-13 Hz (common in the awake state) to theta (θ) waves having a frequency range of 4-7 Hz. Sudden twitch and hypnic jerks, or positive myoclonus, are linked with the onset of sleep during N1. Furthermore, some people may experience hypnagogic hallucinations, and most would lose muscle tone and most conscious awareness of the external environment. 



2. NREM2 (Stage 2: N2 - 45-55% of total sleep in adults) 

Theta wave activity is observed during this stage and it is gradually difficult for sleepers to awaken. Meanwhile, the alpha waves from N1 stage are interrupted by abrupt activity called sleep spindles (thalamocortical spindles) and K-complexes. The frequent range of sleep spindles is 11-16 Hz (commonly 12-14 Hz). During this stage, both muscular activity on the EMG and conscious awareness of the external environment decreases. 

3. NREM3 (Stage 3: N3 - Deep Sleep, Slow-wave Sleep (SWS) - 15-25% of total sleep in adults) 

Source: https://www.sciencedirect.com/science/article/pii/S0896627318300722

https://en.wikipedia.org/wiki/Slow-wave_sleep
NREM3 or SWS is generated by synchronised EEG activity and characterised by slow waves with a frequency range of 0.5-4.5 Hz, relatively high amplitude power with peak-to-peak amplitude greater than 75 μV. 

• The first section of the slow wave represents a "down state", which is an inhibition or hyperpolarising phase that shows neocortex neurons are inactive. 
• The second section of the wave represents a "up state", which is an excitation or depolarising phase that shows neurons fire briefly at a rapid rate. Carlson (2012) observed the principal characteristics during SWS are moderate muscle tone, slow or absent eye movement, and lack of genital activity. 
• Carey (2017) postulated SWS plays a role in memory consolidation, which is referred to as "sleep-dependent memory processing". 
• Walker (2009) found patients suffering from primary insomnia demonstrated impaired memory consolidation, therefore they performed poorly in memory tasks compared to healthy patients who slept adequately. Furthermore, Walker theorised that SWS plays a role in consolidating declarative memory (including semantic and episodic memory). He hypothesised a central model that implied long-term memory storage is facilitated by interactions between the hippocampal and neocortical networks. 
• Gais et al. (2002) found the density of sleep spindles increased in subjects who trained to learn a declarative memory task compared to control subjects who performed similar visual stimulation and cognitively-demanding tasks that didn't require learning. This finding associated with the spontaneously occurring wave oscillations accounting for the intracellular recordings from thalamic and cortical neurons. 
• Rasch (2007) observed the hippocampus reactivates during SWS after the spatial learning task, suggesting its role in spatial declarative memory. Peigneux (2004) remarked a correlation between the amplitude of hippocampal activity during SWS and the improvement in spatial memory performance, such as route retrieval, the day after. 
• SWS is shown to play an exclusive role as a context cue that reactivates the memories and facilitates its consolidation by the hippocampal neurons in response to odour re-exposure and sound associated with previously shown pictures-locations. 
• When emotions with negative salience were presented to subjects as a cue during SWS, it yielded greater reactivation, which implied an enhanced consolidation in comparison to neutral memories. Scott (2014) suggested this effect is predicted by sleep spindles, which discriminates the memory processes during sleep and facilitates emotional memory consolidation. 
• During SWS, levels of cholinergic activity increases disrupts memory processing, which demonstrates its important role in hippocampus-dependent memory consolidation. Since acetylcholine modulates the direction of information flow between the hippocampus and neocortex during sleep, suppressing this neurotransmitter during SWS consolidates sleep-related declarative memory. 
• Studies on sleep deprivation implied that the primary function of SWS is aiding the brain's recovery from its daily activity. Another function of SWS proposed was the secretion of growth hormone, which may account for the decrease in sympathetic and increase in parasympathetic neural activity. 

Describe the EEG characteristics of NREM3

• Iber et al. (2007) found large 75 μV (0.5 - 2.0 Hz) delta waves predominate the EEG in stage NREM3, which is defined by the presence of 20% delta waves in any 30-second epoch of the EEG during sleep. 
• Jones (2003) suggested SWS is likely produced through activation of serotonergic neurons of the raphe system. 
• Williams et al. (1997) observed the slow-wave in the cortical EEG is produced by the thalamocortical communication through the thalamicortical neurons, which is generated by the "slow oscillation". This property depends on membrane potential bistability, which is a property of these neurons due to an electrophysiological component known as "I t window". 
• This window is caused by the overlap underneath activation and inactivation curves plotted for T-type calcium channels (inward current). 
• Multiplying the 2 curves, and superimposing another line on the graph yields a small Ik leak current (outward). This is followed by the interplay between these inward (I t window) and outward small Ik leak), yielding 3 equilibrium points observed at -90, -70 and -60 mV (-70 is unstable, -90 and -60 is stable). This property allows slow waves to be generated due to an oscillation between 2 stable points. 

What are the functions of SWS? 
i. Hemispheric asymmetries in human sleep 

• Some animals, such as fish, dolphins and birds, are able to sleep with only 1 brain hemisphere inactive, leaving the other hemisphere awake to perform normal functions and maintain alertness. This sleep phenomenon is known as 'unihemispheric SWS', which is partially observable in human beings. 
• Kattler et al. (1994) found a unilateral activation of the somatosensorial cortex after applying a vibrating stimulus to a human hand of a sleeping subject. This suggested an inter-hemispheric change during the first hour of non-REM sleep and consequently the presence of a local and use-dependent aspect of sleep. Sekimoto (2000) detected a number of delta waves in the frontal and central regions of the right hemisphere. 
• During SWS, the left hemisphere is predominantly active particularly in the default-mode network. Tamaki et al. (2016) implied this asymmetry is associated with the sleep-onset-latency, a sensitive parameter of the first night effect (i.e. the reduced quality of sleep during the first session in the lab). 
• During the first night, the left hemisphere is demonstrated to be more sensitive to deviant stimuli compared to other nights, indicating asymmetry. This accounts for the decreased sleep in half of the brain during SWS, suggesting the left hemisphere's vigilant role. 
• Moreover, the left hemisphere demonstrated faster behavioural reactivity during SWS of the first night. This rapid reawakening is linked to the regional asymmetry in the neural activities of SWS. Tamaki et al. (2016) observed that the hemispheric asymmetry in SWS is part of a protective mechanism, suggesting higher sensitivity to danger and unfamiliar environments, incentivising vigilance and reactivity during sleep. 

ii. Neural control of SWS 

• Carlson (2012) noted several neurotransmitters are involved in sleep and waking patterns such as acetylcholine, noradrenaline, histamine, orexin and serotonin. 
• Research suggested subjects recalled their dreams at a higher rate during SWS compared to other stages of the sleep cycle, which indicates that mental activity is related to real life events. 

iii. Physical healing and growth 

• SWS is the constructive phase of sleep for the body-mind system to recover every day. During this stage, the food and drink consumed during awakened state are synthesised into complex proteins of living tissue. Furthemore, glial cells within the brain are rejuvenated with carbohydrates to provide energy for the brain. 

iv. Learning and synaptic homeostasis 

• Learning and memory formation typically occurs during the awakened state through a process called long-term potentiation, and SWS is thought to be linked to regulation of synapses thus potentiation. 
• Tononi & Cirelli (2006) found, during SWS, synapses are pruned by keeping the strongly stimulated or potentiated synapses and removing weakly potentiated synapses. This would assist in recalibrating synapses for the next potentiation events during wakefulness and for maintaining synaptic plasticity. 
• Gulati et al. (2017) found reactivation and rescaling of synapses co-occur during SWS. 

What brain regions are known to be active in SWS? 

1. Parafacial zone (GABAergic neurons) - Located within the medulla oblongata 
2. Nucleus accumbens core (GABAergic medium spiny neurons - subset of neurons that expresses both D2-type dopamine receptors and adenosine A_2A receptors, located within the striatum. 
3. Ventrolateral preoptic area (GABAergic neurons) - located within the hypothalamus. 
4. Lateral hypothalamus (Melanin-concentrating hormone-releasing neurons) - located within the hypothalamus. 

(c) REM sleep activity (REM Stage - 20-25% of total sleep in humans)

https://en.wikipedia.org/wiki/Rapid_eye_movement_sleep

Polysomnography of REM sleep, EEG highlighted by red box. Eye movement highlighted by red line.

This phase of sleep in mammals and birds is characterised by random rapid movement of the eyes, and low muscle tone throughout the body, and the propensity of the sleeper to dream vividly. It is occasionally known as paradoxical sleep or desynchronised sleep, due to the physiological similarities to waking states such as rapid, low-voltage desynchronised brain waves. 

Describe the neurophysiology of REM sleep 
a. Electrical activity in the brain 

• REM Sleep is considered "paradoxical" because it shares similarities to wakefulness. Despite the body being immobile, the level of brain activity is synonymous to that during wakefulness, with cerebral neurons firing with the same overall intensity as in awakened state. 
• Brown & McCarley (2008) found the EEG of REM sleep consists of fast, low amplitude, desynchronised neural oscillations (brain waves) that resemble similar patterns during wakefulness, which differ from the slow delta waves pattern of NREM deep sleep. 
• Horne (2013) identified the 3-10 Hz theta rhythm in the hippocampus and 40-60 Hz gamma waves in the cortex. 
• Steriade & McCarley (1990) found the cortical and thalamic neurons in the waking and REM sleeping brain are more depolarised than in the NREM deep sleeping brain. 
• During REM sleep, frontal and posterior areas are less electrically connected in most frequencies, which relates to the chaotic experiencing of dreaming. Nevertheless, Pace-Schott (2011) found the posterior areas are more connected with each other, as are both hemispheres of the brain, especially during lucid dreams. 
• A 2000 study discovered energy consumption in the brain during REM sleep, as measured by oxygen and glucose metabolism, equals or exceeds energy use in wakefulness, whereas it was 11-40% lower in NREM sleep. 

- Brainstem: 

• Neural activity during REM sleep is found to occur in the brainstem, especially the pontine tegmentum and locus coeruleus. Steriade & McCarley (1990) observed REM sleep was interrupted and immediately preceded by pinto-geniculo-occipital (PGO) waves, bursts of electrical activity originating in the brainstem. 
• PGO waves occurred in clusters roughly every 6 seconds for 1-2 mins during the transition from deep to paradoxical sleep. 
• Several studies discovered PGO waves displayed their highest amplitude upon movement into the visual cortex, accounting for the "rapid eye movements" in REM sleep. 

- Forebrain: 

• Matarazzo et al. (2011) found the limbic and paralimbic systems within the forebrain demonstrated more activity than other brain areas. 
• Nofzinger (1997) identified the "anterior paralimbic REM activation area" (APRA) is activated during REM sleep, which is associated with emotion, memory, fear and sex, and possibly the experience of dreaming. 
• The superior frontal gyrus, medial frontal areas, intraparietal sulcus, and superior parietal cortex, and areas involved in sophisticated mental activity, demonstrated equal activity in REM sleep as in wakefulness. 
• Sanford & Ross (2011) identified the amygdala as another region active in REM sleep and produces PGO waves.  

b. Chemicals in the brain 

• Both waking and REM sleep involve higher use of acetylcholine, which may cause the faster brainwaves. During the same stages, the monoamine neurotransmitters noradrenaline, serotonin and histamine are unavailable for the brain to use. 
• Several studies found REM sleep can be used in humans if the monoamine neurotransmitters are depleted and acetylcholinesterase inhibitor and Carbachol were introduced. 
• Luppi et al. (2008) identified orexin and gamma-Aminobutyric acid (GABA) facilitates wakefulness, and depletes during deep sleep, hence inhibits REM sleep. 

- Models of REM regulation 

• In 1975-77, Robert McCarley and Allan Hobson proposed the activation-synthesis hypothesis that suggested control over REM sleep involves pathways of "REM-on" and "REM-off" neurons in the brainstem. 
• REM-on neurons are primarily cholinergic (i.e. releases acetylcholine), while REM-off neurons primarily release serotonin and noradrenaline, which among other functions suppress REM-on neurons. 
• Using Lotka-Volterra equations to describe the following cyclical inverse relationship, McCarley & Hobson (1977) implied that the REM-on neurons stimulate REM-off neurons, as part of a mechanism for the cycling between REM and NREM sleep. 
• Lydic & Baghdoyan (2007) discovered acetylcholine levels were higher in the brainstem during REM sleep. 

c. Eye movements 

• A majority of eye movements in REM sleep are apparently less rapid than those normally exhibited by awake humans. Moreover, they are shorter in duration and likely to loop back to their starting point, with an estimated 7 loops occurring over 1 minute of REM sleep. 
• Steriade & McCarley (1990) found the eyes drifted apart in SWS, though the eyes tend to move in tandem in REM sleep. A 2010 study found these eye movements follow the PGO waves originating in the brainsteam. 
• Although congenitally blind people typically lack visual imagery in their dreams, their eyes still move in REM sleep, nonetheless. 
• Zhang (2016) suggested the functional purpose of REM sleep is for processing memory processing, and REM is a side effect of the brain processing the eye-related procedural memory. 

d. Circulation, respiration and thermoregulation 

• Parmeggiani (2011) observed irregularities in heart rate, cardiac pressure, cardiac output, arterial pressure and breathing rate as the brain transitions into REM sleep. This suggests homeostasis is suspended and respiratory reflexes (e.g. response to hypoxia) is diminished during REM sleep. 
• Overall, the brain exerts less homeostatic control over breathing, and the lungs aren't influence of electrical stimulation of respiration-linked brain areas as it usually would in NREM sleep and in wakefulness. Furthermore, the fluctuations of heart rate and arterial pressure coincided with PGO waves and REM, twitches, or sudden changes in breathing. 
• Jouvet (1999) found penile erections (nocturnal penile tumescence or NPT) normally accompany REM sleep in rats and humans. Even if a male has erectile dysfunction (ED) during wakefulness, they can experience NPT episodes during REM, implying that ED is due to a psychological rather than a physiological cause. In females, clitorial erections (noctural clitoral tumescence or NCT) leads to enlargement, accompanied by vaginal blood flow and transudation (i.e. lubrication). Brown at al. (2012) estimated the penis and clitoris are erected between 1 and 3.5 hours during REM sleep. 
• Since body temperature is not efficiently regulated during REM sleep, organisms demonstrate additional sensitivity to temperatures outside their thermoneutral zone. Parmeggiani (2011) noticed cats and other small furry mammals shivered and breathed swiftly in order to regulate body temperature during NREM sleep but not during REM sleep. Moreover, the loss of muscle tone compromised the sleeping animal's ability to regulate temperature through body movement. He found that neurons that typically fire in response to cold temperatures, hence triggering neural thermoregulation, didn't seem to fire during REM sleep, as they do in NREM sleep and wakefulness. 
• A 1999 study suggested hot or cold environmental temperatures decreases the proportion of REM sleep, as well as the total amount of sleep, consequently. Parmeggiani (2011) explained an organism would not experience REM sleep if its thermal indicators drop outside of a certain range at the end of deep sleep stage. 

e. Muscle 

• REM atonia is defined as virtually complete paralysis of the body during REM sleep, which is achieved by inhibition of the motor neurons. 
• When the body transitions into REM sleep, motor neurons throughout the body hyperpolarise. Since their negative membrane potential decreases by 2-10 mV, therefore increasing the threshold in which a stimulus must surpass to excite the neuron. 
• Steriade & McCartley (1990) suggested muscle inhibition may be caused by unavailability of monoamine neurotransmitters and perhaps by mechanisms associated with waking muscle inhibition. Lai & Siegel (1999) stated that the medulla oblongata, located between pons and spine, demonstrated capacity for organism-wide muscle inhibition. Other muscle features during REM sleep include localised twitching and reflexes, and pupil contrition.

Describe the neuropsychology of REM sleep 
a. Dreaming 

• Solms (1997) found that 80% of neurotypical people woken up during REM sleep stage provided a vivid dream report. Furthermore, research has found that those dream reports were longer, contained more narrative descriptions of the dreams experienced, and subjects awakened from REM estimated the duration of their dreams to be longer. 
• LaBerge (1992) reported lucid dreams occurred more regularly in REM sleep compared other sleep stages. 
• A 2000 study observed the mental events occurring during REM contained certain dream hallmarks including narrative structure, convincingness (experiential resemblance to waking life), and incorporation of instinctual themes. In addition, the elements of the dreamer's recent experience may have originated from episodic memory. 
• Hobson and McCarley (1977) proposed the PGO waves characteristic of "phasic" REM may electrically excite the visual cortex and forebrain, amplifying the hallucinatory aspects of dreaming. 
• A 1992 study outlined a lack of bizarre dreams reported by people awakened during phasic REM sleep, compared to tonic REM sleep. Furthermore, Reinsel, Antrobus & Wollman postulated a possible relationship between the 2 phenomena that suggested the higher threshold for sensory interruption during REM sleep, facilitating the brain to travel further along unrealistic and peculiar trains of thought. 

b. Effects of SSRIs 

• A 2013 study demonstrated selective serotonin reuptake inhibitors (SSRIs) elicited a marked effect on REM sleep neurobiology and dreaming, which served to intensify dreaming in humans. 
• A 2001 study described SSRI treatment (e.g. paroxetine, fluvoxamine) reduced the average amount of dream recall frequency compared to baseline measurements caused by serotonergic REM suppression. It indicated fluvoxamine increased the length of dream reporting, strangeness of dreams and intensity of REM sleep, all of such effects peaked during acute discontinuation than treatment or baseline days. However, the subjective intensity of dreaming increased and the tendency to experience REM sleep decreased during SSRI treatment compared to baseline and discontinuation days. 

c. Creativity 

• Rasch & Born (2013) found people awakened during REM sleep performed exceptionally on tasks such as anagrams and creative problem solving, suggesting the brain's increased reception to semantic priming effects. 
• Wagner et al. (2004) argued that REM sleep facilitates creativity to form associative elements into novel useful combinations. 
• Cai et al. (2009) implied this creativity is attributed to changes during NREM sleep in cholinergic and noradrenergic neuromodulation. 
• Hasselmo (1999) explained high levels of acetylcholine in the hippocampus suppresses feedback from the hippocampus to the neocortex, while lower levels of acetylcholine and noradrenaline in the neocortex promoted the uncontrolled speed of associational activity. This contrasts the finding during waking consciousness, where higher levels of acetylcholine and noradrenaline inhibit recurrent connections in the neocortex. 
• Cat et al. (2009) suggested REM sleep via this process includes creativity by "allowing neocortical structures to reorganise associative hierarchies, in which information from the hippocampus is reinterpreted relative to previous semantic representations or nodes". 

Discuss the timing of REM sleep 

This hypnogram illustrates sleep architecture from midnight to 6:30 am, with deep sleep early on. Notice there is more REM (marked red) before waking. Current hypnograms reflect the recent decision to combine NREM stages 3 and 4 into a single stage 3.

• In the ultradian sleep cycle, an organism alternates between deep sleep (slow, large, synchronised brain waves) and REM sleep (faster, desynchronised waves). It's known sleep occurs as part of the larger circadian rhythm, which impacts on sleepiness and physiological factors based on timekeepers within the body. 
• Parmeggiani (2011) outlined the distribution of sleep throughout the day or clustered during a segment of the rhythm: in nocturnal animals, during the daytime, and in diurnal animals, at night. Virtually immediately after REM sleep ends, the organism returns to homeostatic regulation. 
• During a night of sleep, humans typically experience about 4-5 periods of REM sleep, which are shorter (~ 15 mins) at the start of the night and longer (~25 mins) towards the end of the night. Many animals and some humans are inclined to wake up, or experience a period of light sleep, for a brief time immediately after a stint of REM sleep. Since the relative amount of REM sleep varies substantially with age, a newborn baby experiences 80% of the total sleep in REM stage. 
• REM sleep normally takes up 20-25% of total sleep in adult humans i.e. Roughly 90-120 mins of an 8 hour night's sleep. McCarley (2007) estimated the first REM episode occurs around 70 minutes after the human falls asleep, followed by cycles of around 90 mins, with each cycle including a larger proportion of REM sleep. Aeschbach (2011) implied the increased REM sleep later in the night is associated with the circadian rhythm, which also occurs in people who didn't sleep in the first segment of the night. 
• Frank (2011) outlined that in the weeks after a human baby is born, during which its nervous system develops, their neural patterns in sleep demonstrate a rhythm a REM and NREM sleeps. This phenomenon also occurs in faster-developing mammals, but in utero. Infants tend to spend a major proportion of their sleep time in REM than adults, which decreases considerably in childhood. Although older adults sleep less overall, their REM sleep absolute time doesn't tend to vary considerably, thus they spend a large proportion of sleep in REM. 
• Kryger & Roth (2000) subclassified REM into tonic and phasic modes. Tonic REM is characterised by theta rhythms in the brain, while phasic REM is characterised by PGO waves and actual "rapid" eye movements. Emmis, Krakow & Voss (2010) discovered the processing of external stimuli is inhibited during phasic REM, increasing the difficulty of sleepers becoming aroused from phasic REM in SWS. 

What are the possible functions of REM sleep?
The precise function of REM sleep is poorly understood, and there is debate between researchers regarding its role in animal survival and behaviour. 

i. Memory 

• REM sleep is theorised to aid in the preservation of certain types of memory: specifically, procedural memory, spatial memory, and emotional memory. Rats were observed to spend more time in REM sleep after intensive learning. 
• Rasch & Born (2013) observed experimental REM sleep occasionally inhibited memory consolidation in rats, especially complex processes such as escaping from a labyrinth. In humans, there is evidence for REM sleep associating with memory improvements pertaining to learning of new procedures, i.e. new ways of manoeuvring the body, and new techniques of problem solving. 
• Insufficient REM sleep correlated with impaired declarative (factual) memory only in more complex cases, such as memories of detailed stories occurring in the past. It is suggested that REM sleep counteracts the suppression of certain thoughts, but the mechanism of such process is unknown. 
• The 'dual-process hypothesis' of sleep and memory proposed 2 major phases of sleep that correspond to different types of memory. "Night half" studies found SWS, part of NREM sleep, plays a role in forming declarative memory. 
• Marshall et al. (2006) found artificial augmentation of NREM sleep improves the following-day recall of memorised pair of words. 
• Tucker et al. (2006) showed that a daytime nap consisting of merely NREM sleep augments declarative memory but not procedural memory. 
• The 'sequential hypothesis' postulates the REM and NREM sleeps cooperate to consolidate memory. 
• Siegel (2001) observed that extreme REM sleep deprivation doesn't significantly interfere with memory. One case study of a subject who had little or no REM sleep due to a shrapnel injury to the brainstem didn't suffer from memory impairment. Though antidepressants REM sleep, it didn't significantly impair memory and perhaps augment it. 
• In 1983, Mitchison and Crick proposed that by virtue of its inherent spontaneous activity, the function of REM sleep involves removing particular unwanted modes of interaction in networks of cells in the cerebral cortex, a process they characterised as "unlearning". This would lead to strengthening of relevant memories, and disintegration of weaker, transient, "noise" memory. 
• Datta (1999) explained that since PGO electrical waves preceded the eye movements, it impacted on memory. Parmeggiani (2011) suggested that REM sleep provided opportunities for "unlearning" to occur in the basic neural networks involved in homeostasis, which are protected from "synaptic downscaling" during deep sleep. 

ii. Neural Ontogeny 

• The ontogenetic hypothesis postulates that REM sleep (or active sleep in neonates) facilitates the developing brain through neural stimulation to create mature neural connections. 
• Mirmiran et al. (1983) demonstrated that REM sleep deprivation early in life lead to behavioural problems, permanent sleep disruption, and decreased brain mass. 
• Frank (2011) found REM deprivation affects the development of the visual system, particularly in the lateral geniculate nucleus and primary visual cortex. 

iii. Defensive immobilisation 

• Tsoukalas hypothesised that REM sleep is an evolutionary modification of the tonic immobility reflex, also known as animal hypnosis or death feigning. This familiar defence mechanism functions as the last line of defence against an attacking predator, which completely immobilises the animal to give off the impression of being dead. Tsoukalas claimed the neurophysiology and phenomenology of this reaction shared similarities to REM sleep. For instance, both reactions manifests brainstem control, paralysis, hypocampal theta rhythm, and thermoregulatory adjustments. 

iv. Shift of gaze 

• The "scanning hypothesis" suggests the directional properties of REM are associated to a shift of gaze in dream imagery. However, evidence against this hypothesis is that such eye movements occur in the congenitally blind in spite of lack of vision. Moreover, binocular REMs are non-conjugated (i.e. both eyes don't face in the same direction at a time), hence lack a fixation point. 
• A 2010 study found, in the context of goal-oriented dreams, the eyes gaze towards the dream action, which ascertains from associations in the eye and body movements of REM sleep behaviour disorder patients who act out their dreams. 

v. Oxygen supply to cornea 

• Maurice (1998) proposed an association between REM sleep with oxygen supply to the cornea, as well as the speculation of aqueous humour (the liquid between cornea and iris) being stagnant if infrequently stirred. He evaluated that oxygen from the iris needed to reach the cornea by diffusion through stagnant aqueous humour, which he deemed inadequate. 
• The theory states that an awake animal's eye movements (or cool environmental temperature) allows the aqueous humour to circulate. Moreover, an animal in REM sleep provides the crucial stirring of the aqueous humour. This hypothesis is supported by the finding of foetuses, as well as eye-sealed newborn animals, spending a large proportion of time in REM sleep. However, evidence disputing this theory is owls not moving their head more in REM sleep compared to NREM sleep, since their eyes are virtually immobile. 

vi. Other theories 

• There is a suggestion that monoamine shutdown is needed for the recovery of monoamine receptors back to maximum sensitivity. 
• In 1966, Frederick Snyder proposed the 'sentinel hypothesis' of REM sleep, based on his observation of REM sleep in several mammals,  such as the rat, the hedgehog, the rabbit, and the rhesus monkey, always followed by a brief awakening. 
• Snyder hypothesised that REM sleep activates an animal periodically to allow them to scan the environment for predators. However it fails to explain the muscle paralysis phenomenon of REM sleep. Studies used logical analysis to imply that the muscle paralysis occurs to prevent the animal from fully waking up unnecessarily, which allow it to easily return to deeper sleep. 
• Horne (2013) suggested that REM sleep in modern humans compensates for the decreased demand for wakeful food foraging. 
• Other theories include REM sleep regulating the warm temperature of the brain, stimulating and stabilising the neural circuits that are deactivated during wakefulness, creating internal stimulation to facilitate development of the CNS. 

What are the different types of brain waves? 

1. Alpha (α) waves 
https://en.wikipedia.org/wiki/Alpha_wave

• They are neural oscillations in the frequency range of 8-12 Hz produced by the synchronous and in phase or constructive electrical activity of thalamic pacemaker cells in humans. 
• They are present at different stages of the sleep-wake cycle. Alpha activity during the relaxed mental state (at rest with eyes closed, but not tired or asleep) is focused in the occipital lobe, with speculative origins in the thalamic regions. 
• Niedermeyer (1997) detected alpha waves in infants around 4 months old, which was initially a frequency of 4 waves per second. By age 3, the mature alpha wave is established at 10 waves per second. 
• Alpha wave activity also occurs during REM sleep, located mainly in a frontal-central regions of the brain. However, the main function of alpha activity during REM sleep is poorly understood. 

2. Beta (β) waves 

https://en.wikipedia.org/wiki/Beta_wave

They are neural oscillations in the frequency range of 12.5-30 Hz. They are separated into 3 segments: 

- Low (Beta 1 power) = 12.5 - 16 Hz 

- Normal (Beta 2 power) = 16.5 - 20 Hz 

- High (Beta 3 power) = 20.5 - 28 Hz 

• Baumeister et al. (2008) associated low beta waves with active, busy or anxious thinking and active concentration. 
• Baker (2007) suggested beta waves produced by the motor cortex associates with the muscle contractions that occur in isotonic movements and become suppressed prior to and during movement changes. 
• Lalo et al. (2007) associated bursts of beta activity with reinforcement of sensory feedback in static motor control, which decreases during movement change. 
• Zhang et al. (2008) found beta activity increases during resistance and voluntary suppression of movement. 
• Yaple et al. (2018) uncovered 2 distinct beta components of reward feedback: a high beta (low gamma) component and a low beta component. 
• Haji-Hosseini et al. (2012) observed the high beta component was more prominent upon receipt of an unexpected outcome, with a low probability, relating to unexpected gains. However, the low beta component associates with the exclusion of gains, which were expected. 

Function of beta waves: 

• Beta waves indicate inhibitory cortical transmission mediated by gamma amino butyric acid (GABA), the principal inhibitory neurotransmitter of the mammalian nervous system. 
• Researchers found benzodiazapines and drugs that modulate GABA_A receptors trigger the generation of beta waves in EEG readings in humans and rats. 
• Children diagnosed with duplication 15q11.2-q13.1 syndrome (Dup15q) who have duplications of GABA_A receptors subunit genes GABRA5, GABRA3 and GABRA6 demonstrated diffuse spontaneous beta waves in scalp EEG recordings. 
• Hipp et al. (2019) found children with Angelman syndrome, with deletions of the same GABA_A receptor subunit genes, present reduced beta amplitude. 
• It's implied beta waves are likely biomarkers of GABAergic dysfunction, especially in neurodevelopment disorders caused by 15q deletions / duplications. 

3. Delta (δ) waves 

https://en.wikipedia.org/wiki/Delta_wave

• They are a high amplitude brain wave with a frequency range of 0.5 - 4 Hz, recorded on an EEG around the deep stage 3 of NREM sleep as well as stage 4 sleep. 
• Gennaro et al. (2000) associated delta waves with K-complexes, which precede them. Brigo (2011) classified delta waves according to the location of activity into frontal (FIRDA), temporal (TIRDA), and occipital (OIRDA) intermittent delta activity.  
• Ehlers & Kupfer (1997) found females exhibited more delta wave activity than males, which is observed in most mammal species. However, this discrepancy doesn't become obvious until early adulthood (in the 30s or 40s in humans), with males exhibiting greater age-related diminishes in delta wave activity than females. 
• Gross (1992) stated delta waves arose either in the thalamus or cortex, associated with the coordination of reticular formation. 
• Mistlberger et al. (1987) demonstrated the suprachiasmatic nuclei (SCN), in the cortex, regulated delta waves. Furthermore, those waves exhibited a lateralisation, with right hemispheric dominance during sleep. 
• Lee et al. demonstrated T-type calcium channels mediated delta waves, and Hobson & Pace-Schott (2002) observed GABA globally inhibit neurons during delta wave sleep. 
• It is known delta wave activity stimulates the release of several hormones such as growth hormone release hormone (GHRH) and prolactin (PRL). GHRH is secreted from the hypothalamus, which subsequently stimulates release of growth hormone (GH) from the pituitary. Meanwhile, secretion of PRL is regulated by the pituitary. Brandenburger (2003) observed the release of thyroid-stimulating hormone (TSH) decreases in response to delta-wave signalling. 

4. Gamma (γ) waves 

https://en.wikipedia.org/wiki/Gamma_wave

• This wave pattern has a frequency range of 25 - 150 Hz, with 40 Hz being of great interest to researchers. 
• Gamma waves were initially recorded from the visual cortex in awake monkeys, before subsequent recordings in the premotor, parietal, temporal, and frontal cortical regions. 
• McCormick et al. (2015) observed the gamma waves are produced by a group of neurons in the cortico-basal ganglia-thalami-cortical loop. 
• van Kerkoerle et al. (2014) implied this activity demonstrates feedforward connections between distinct brain regions, in contrast to alpha wave feedback across the same regions. 
• Le Van Quyen et al. (2016) demonstrated gamma waves associated with the firing of single (mainly inhibitory) neurons during all states of the sleep-wave cycle. 
• The mechanisms and substrates behind the role of gamma activity in the generation of different states of consciousness remains unknown. 
• Some researchers debated the validity or significance of gamma wave activity detected by scalp EEG, since the frequency band of gamma waves overlaps wit hthe electromyographic frequency band. Muthukumaraswamy (2013) highlighted the muscle activity contaminates gamma signal recordings. 
• Research employed local muscle paralysis techniques to confirm that EEG recordings comprised of a EMG signal, which are traced back to local motor dynamics such as saccade rate or other motor actions performed by the head. Muthukumaraswamy suggested the EMG artefacts can be minimised by advances in signal processing and separation, including the application of independent component analysis or techniques based on spatial filtering. 

Function of gamma waves: 

I. Conscious perception 

• Studies suggested gamma waves plays a role in the formation of coherent, unified perception, known as the problem of combination in the binding problem, as the neural firing rates are apparently synchronised across different brain regions. 
• Gold (1999) hypothesised the 40-Hz gamma waves associates with visual consciousness, since 2 neurons oscillate synchronously when a single external object stimulates their respective receptive fields. 
• Crick & Koch (1990) claimed a correlation between the binding problem and the problem of visual consciousness, and a causal implication for synchronous 40 Hz gamma waves in visual awareness and visual binding. 
• Llinás experimentally supported a hypothesis that the basis of consciousness in awake states and dreaming is 40-Hz gamma waves throughout the cortical mantle in the form of thalamocortical repetitive activity. Furthermore, he proposed that the concurrent summation of specific and non-specific 40-Hz activity long the radial dendritic axis of particular cortical elements facilitates the coexistence into a single cognitive event. In addition, the brainstem may modulate the resonance of gamma waves and provides sensory input in the awake state and intrinsic activity during dreaming. 
• Llinás' thalamocortical dialogue hypothesis for consciousness implied that the 40-Hz gamma waves observed in wakefulness and in dreaming is a product of cognition, evolved from coherent 40-Hz resonance between thalamocortical-specific and non-specific loops. 
• Llinás & Ribary (1993) advocated that the specific loops generated the content of cognition, and a non-specific loop produced the temporal binding required for the integration of cognitive experience. 
• Engel et al. (1999) debated that temporal synchrony was the basis of consciousness, by expounding the gamma wave hypothesis as "the synchronisation of neuronal discharges serving for the integration of distributed neurons into cell assemblies, which may underlie the selection of perceptually and behaviourally relevant information." 

II. Attention 

• Pollack (1999) proposed a mechanism that gamma waves, produced by the thalamus, associated with neural consciousness through the mechanism for conscious attention. 
• He claimed the synchronised oscillation of neuronal clusters during these brief periods of synchronised firing generate memories and associations from the visual percept to other notions. He theorised this phenomenon combines a network of cognitive processes to produce an organised, synchronised cognitive act, such as perception. 
• Buzsaki (2006) theorised an association between gamma waves and a solution to the binding problem. 
• A handful of studies observed gamma waves as neural synchrony from visual cues in both conscious and subliminal stimuli. Ward et al. (2006) suggested these findings may clarify how neural synchrony associates with stochastic resonance in the nervous systems. 

5. Theta (θ) waves

https://en.wikipedia.org/wiki/Theta_wave


This neural oscillatory pattern recorded on a scalp EEG has a frequency of 4-7 Hz. There are 2 types of theta rhythms: hippocampal & cortical. In the 1950s, the hippocampal theta rhythm was first observed in the hippocampus and other brain structures in mammal species such as rodents, rabbits, dogs, cats, bats, and marsupials. Cortical theta rhythms are low-frequency components of scalp EEG, typically recorded from humans. The term "theta" has 2 interpretations: 

1. A specific type of regular oscillation observed in the hippocampus and several other brain regions linked to it, in mice or rats. 
2. EEG oscillations in the 4-7 frequency range, regardless of its origins in the brain or their actual functional significance, in humans. 

I. Hippocampal theta rhythms 

Sample mouse EEG - Theta rhythm is prominent during part of awakening and REM sleep. 

• Due to the density of its neural layers, the hippocampus produces large EEG signals in the 4-10 Hz range, lasting many seconds. This wave pattern is referred to as the 'hippocampal theta rhythm'. 
• Vanderwolf (1969) reported the hippocampal theta had 2 conditions: (1) When an animal is running, walking or other active interactions with its surroundings, and (2) during REM sleep. 
• At normal running speed, theta wave frequency is about 6.5 Hz, whereas at the fastest running speeds, the frequency increases to 9 Hz. 
• Sainsbury et al. (1987) observed theta waves in rats, cats and rabbits during states of motionless alertness and arousal. 

i. Type 1 and Type 2 

• Kramis et al. (1975) proposed 2 distinct types of hippocampal theta rhythm in rats, with different behavioural and pharmacological attributes. 
• Type 1 theta is known as "atropine resistant", which appears during locomotion and other types of "voluntary" behaviour and during REM sleep. It has a frequency of roughly 8 Hz, and is unaffected by the anticholinergic drug atropine. 
• Type 2 theta is known as "atropine sensitive", which appears during both immobility and anaesthesia triggered by urethane. It has a frequency range of 4-7 Hz, and is cancelled out by atropine. 

ii. Relationship with behaviour 

• Vanderwolf (1969) argued that hippocampal theta rhythms can be predicted based on the animal's movements, rather than the reasons behind such movements. 
• Examples of active movements include running, jumping, bar-pressing, or exploratory sniffing, which are suggested to be strongly associated with theta rhythms. 
• Examples of inactive movements such as eating or grooming, which are associated with LIA. 
• Whishaw & Vanderwolf (1973) demonstrated theta rhythms frequently initiate several 100 ms before the onset of movement, corresponding with the intention to move instead of with feedback produced by movement. 

iii. Mechanisms 

• Stewart & Fox (1990) observed the medial septa area is primarily involved in the generation of hippocampal theta rhythms. This brain area includes the medial septal nucleus and the vertical limb of the diagonal band of Broca, but the lateral septal nucleus is not involved. 
• Buzsáki (2002) uncovered a number of connections between the medial septa area and a number of brain regions involved in theta wave modulation, such as the hippocampus, entorhinal cortex, perirhinal cortex, retrosplenial cortex, medial mammillary and supramammillary nuclei of the hypothalamus, anterior nuclei of the thalamus, amygdala, inferior colliculus, and some brainstem nuclei. 
• Ujfalussy & Kiss (2006) found projections from the medial septa area are cholinergic, GABAergic or glutamatergic. They argued GABAergic or glutamatergic signals play an important role in theta rhythms, since the cholinergic receptors doesn't respond adequately rapid in the production of theta waves. 
• More research is required to understand the type 1 and type 2 theta waves on different pacemakers. Kirk (1998) theorised the supramammilary nucleus of the hypothalamus exhibits control in theta 2 rhythms, while Wang (2002) proposed a hypothesis of a feedback loop involving the medial septa area and hippocampus influencing the type 1 theta wave frequency. 
• Alonso & Llinás (1989) identified sodium-dependent voltage-dependent oscillations in membrane potential at near-action potential voltages within hippocampal and entorhinal neurons. 
• Buzsáki (2002) observed oscillations in neurons of the CA1 and dentate gyrus were caused by an interplay of dendritic excitation via a persistent sodium current with perisomatic inhibition. 

iv. Research findings 

• Vertes (2005) discovered some short-term memory tasks resulted in theta wave activity arising from the hippocampus. 
• Buzsáki (2002) implied these rhythms illustrated the "on-line" state of the hippocampus, suggesting its preparedness to process signal inputs. 
• Vandewolf (1969) found an association between theta rhythms and various voluntary behaviours (e.g. exploration, spatial navigation, etc.) and alert states (e.g. goosebumps) in rats, implying its role in processing of sensory information, spatial learning and navigation. 
• Hyman et al. (2003) demonstrated that both synaptic plasticity and strength of electrical inputs to the hippocampal region CA1 vary systematically with continuous theta rhythms. 

6. Mu (μ) waves 

https://en.wikipedia.org/wiki/Mu_wave

• Also known as comb or wicket rhythms, arciform rhythms, or sensorimotor rhythms, mu rhythms are synchronised patterns of electrical activity exerted by neurons in brain regions responsible for voluntary movement. 
• Their frequency range is 7.5-12.5 Hz (primarily 9-11 Hz), mainly produced by the motor cortex, in a band approximately from ear to ear. 
• When mirror neurons fire, mu waves are found to be suppressed in monkeys. Researchers think mirror neurons exist in certain brain regions such as the right fusiform gyrus, left inferior parietal lobule, right anterior parietal cortex, and left frontal gyrus. 
• Pineda (2005) suggested that mu wave suppression indicated levels of global activity, detectable in the frontal and parietal networks. Moreover, he found suppression of the resting oscillation during the observation of sensory information such as sound or sight, typically within the frontoparietal (motor) cortical regions. 
• Berchicci etal. (2011) detected mu waves in infants as early as 4-6 months, with a peak frequency of 5.4 Hz, which rapidly increases later in life, reaching 7.5 Hz by 2 years. The peak frequency of the mu wave continues to increase with age until adulthood, approaching its final and stable frequency of 8-13 Hz. These waves are detected around the central sulcus, typically within the Rolandic cortex. 
• Marshall et al. (2011) thought mu waves indicated an infant's developing ability to imitate as part of its development of motor skills, tool use, and its understanding of causal information through social interaction. 
• Nyström et al. (2011) observed a higher degree of desynchronisation of mu waves in infants compared to adults during actions oriented by goals. This leads to the rationale that mu waves provide clues to the mechanisms shared between action perception and execution in children's language development within the brain. 

https://en.wikipedia.org/wiki/Sleep_spindle
https://en.wikipedia.org/wiki/K-complex
7. Sleep Spindles = They are bursts of neural oscillatory activity generated by reciprocation of the thalamic reticular nucleus (TRN) and other thalamic nuclei during  NREM2 sleep stage in a frequency range of 11-16 Hz with a duration of 0.5 seconds or longer (0.5-1.5 seconds). Pinault (2004) found these spindles are sustained and relayed to the cortex by thalamo-thalamic and thalamo-cortical feedback loops regulated by both GABAergic and NMDA-receptor mediated glutamatergic neurotransmission. 

• Although the function of sleep spindles is unclear, scientists theorised they actively participate in the consolidation of overnight declarative memory through the reconsolidating process. Studies found the density of spindles increase after substantial learning of declarative memory and the degree of increase in stage 2 spindle activity correlates with improved memory performance. 
• Holz et al. (2012) discovered spindles facilitate somatosensory development, thalamocortical sensory gating, synaptic plasticity, and offline memory consolidation.
• Lüthi (2014) found sleep spindles directly modulate interactions between the brain and its external environment, essentially modulating responsiveness to sensory stimuli during sleep. 
• Dang-Vu et al. (2011) found spindles distort the transmission of auditory information to the cortex by isolating the brain from external disturbances during sleep. They also found an association between the amount of brainwave activity in the thalamus and a sleeper's ability to maintain tranquility. 
• Rihm et al. (2014) discovered sleeping organisms re-exposed to olfactory cues would initiate reactivation of those spindles, leading to long-term memory consolidation and improvements in recall performance. 
• Harney (2009) and Dingfelder (2006) found spindles occur in the brain as a burst of activity immediately after muscle twitching during sleep. They suggested, at this moment (particularly in children), the brain is learning about the nerves controlling specific muscles during sleep. 
• Saletin, Goldstein & Walker (2011) found a correlation between sleep spindle activity and integration of new information into existing knowledge as well as controlled remembering and forgetting (fast sleep spindles). 
• Ferrarelli et al. (2007) found the absence of the normal pattern of slow and fast sleep spindles during NREM2 in the brains of schizophrenia patients. Moreover, Niedermeyer & Ribeiro (2000) discovered the lack of sleep spindles was a prominent feature of familial fatal insomnia, a prion disease. 

8. K-complex = A waveform seen on an EEG during NREM2 sleep stage, first discovered by Alfred Lee Loomis in 1937. 

• It consists of a brief negative high-voltage peak, usually greater than 100 μV, followed by a slower positive complex around 350 and 550 ms, and finally a negative peak at 900 ms. They occur approximately every 1.0-1.7 minutes and are often followed by bursts of sleep spindles. Studies found K-complexes occur spontaneously, in response to external stimuli such as sound and physical touch on the skin and inspiratory interruptions. Cash et al. (2009) found many cortical regions generate K-complex, predominately over the frontal areas of the brain. 
• K-complexes are generated by numerous cortical areas of outward dendritic currents from the middle (III) to the upper (I) layers of the cerebral cortex. This coincides with a decrease in broadband EEG power including gamma wave activity. Cash et al. (2009) found these phenomena creates "down-states" of neuronal silence that decreases neural network activity. 
• Amzica & Steriade (1998) found the K-complex activity transfers to the thalamus where it synchronises with thalamocortical network during sleep, generating sleep oscillations such as spindles and delta waves. Cash et al. (2009) observed this finding is identical in the "laminar distributions of transmembrane currents" to the slow waves of slow-wave sleep. 
• Halász (2005) suggested K-complexes play a role in both protecting sleep and engaging in information processing, which are essential components of synchronisation of NREM sleep, and important for responding to both the internal and external stimuli in a reactive manner. This helps in consistently suppressing cortical arousal in response to stimuli that the brain requires in order to initially determine its danger level. 
• Tononi & Cirelli (2006) suggested K-complexes promote the activation homeostasis of synapses and memory consolidation. The activation thresholds of cortical synapses decreases during wakefulness as they processes information, increasing their responsiveness, thus need to be adjusted back in order to preserve their signal-to-noise ratio. 
• The down-state provided by K-complexes decreases the strength of synaptic connections that occur during awakened state in order to preserve the signal-to-noise ratio. Furthermore, as the down-state diminishes, "cortical firing 'reboots' occur in a systematic order" for the repetition and consolidation of memory engrams to be encoded during neuronal firing. 
• K-complexes are present in 5-month-old infants during sleep, which develop with age. Then a faster negative component appears and continues to increase until adolescence in children aged between 3 and 5 years old. Wauquier (1993) noticed the frequency and amplitude of K-complexes is higher in adults younger than 30 years of age compared to older adults above 50 years of age. This corresponds with the reduction of other components of sleep such as sleep spindle density and delta power. 

9. Ponto-geniculo-occipital waves 

https://en.wikipedia.org/wiki/PGO_waves
https://www.frontiersin.org/articles/10.3389/fnhum.2017.00089/full

Ponto-Geniculo-Occipital (PGO) waves in cats - Measurements of single non-bursting type PGO on-state Neuron, located in the caudolateral perobrachial area (C-PBL) of a cat. Widespread distribution of PGO wave activity correlates with a triggering-neuron transitioning from tonic to high frequency activity. 

They are distinctive wave forms of propagating activity between 3 brain regions: the pons, lateral geniculate nucleus (LGN), and occipital lobe, i.e. phasic field potentials. The neurons from the pons branch out in a network transmitting the phasic electrical signal toward the LGN and occipital lobe. There are 2 types of neuronal groups within this network: 
I. Executive Neurons 
These neurons generate and propagate the PGO waves throughout the brain. They are all located in the peribrachial area, which surround the superior cerebellar penduncle. Datta (1997) classified executive neurons into 2 subsets: triggering neurons and transfer neurons. 

1. Triggering Neurons = Located in the caudolateral region of the peribrachial area, these neurons actively fire during NREM sleep especially stage 3. They also fire during REM sleep, but at significantly subdued amplitudes. 
2. Transfer Neurons = These neurons transmit PGO waves from the pons to other regions of the brain residing on the rostral portion of the peribrachial area. Williams & Reiner (1983) categorised its firing patterns into 2 modes. The 1st mode is burst firing through low-threshold Calcium (Ca²⁺) ion channels. The other mode is a repetitive tonic firing through Sodium (Na⁺) dependent ion channels. 

II. Modulatory Neurons 
These neurons fires both excitatory and inhibitory inputs to control the spread of PGO waves transmitted by executive neurons. The inhibitory inputs are important in regulating and controlling the amplitude and frequency of the wave. The following neuron groups and brain regions contribute to the control mechanism. 

1. Aminergic neurons = They release monoamines such as serotonin, dopamine and noradrenaline to suppress the PGO wave amplitudes during periods of wakefulness. 
2. Cholinergic neurons = They release acetylcholine to facilitate PGO wave generation, thus acting as an excitatory neuromodulator for triggering neurons. 
3. Nitroxergic neurons = They release nitrous oxide to trigger the generation of PGO waves, thus acting as an excitatory neuromodulator. 
4. GABA-ergic neurons = They release GABA to inhibit aminergic neurons, hence inhibitory to PGO wave propagation. 
5. Vestibular nuclei = Neurons within the vestibular nuclei transmit excitatory inputs to generate PGO waves when stimulated. 
6. Amygdala = Neurons from the amygdala transmit excitatory inputs to generate PGO waves when stimulated. 
7. Suprachiasmatic nuclei = Neurons from the SCN act to regulate REM sleep 
8. Auditory stimulation = Callaway et al. (1987) found auditory stimulation increases PGO waves during waking and sleeping cycles with neurons associated with auditory information transfer. Bowker & Morrison (1977) observed auditory stimulation increased the amplitude of PGO waves in SWS, but didn't decrease the amplitude of PGO waves with repeated auditory stimulation. Datta (1997) proposed a positive-feedback mechanism that may be excited by evoked PGO waves as part of PGO wave generation due to auditory stimulation. 
9. Basal ganglia = The subthalamic nucleus (STN) is reciprocally linked to the PGO-transferring nuclei of the pons. A 2009 study found PGO-liked waves transmitted by the STN in humans exhibit similar wave patterns to the PGO waves recorded in cats, during pre-REM and REM sleep stages. Fernández-Mendoza et al. (2009) suggested the STN plays a role in an ascending activating network involved in the rostral transmission of PGO waves during REM sleep in humans. 

10. Sensorimotor rhythm (SMR) 
https://en.wikipedia.org/wiki/Sensorimotor_rhythm

This brain wave is an idle oscillatory rhythm of synchronised electrical activity in the frequency range of 13-15 Hz. 
So far, the significance of SMR is poorly understood. It is known a person's corresponding sensorimotor areas being idle (e.g. during states of immobility) associated with stronger SMR amplitudes. A 1993 study found SMR amplitude decreases upon activation of the corresponding sensory or motor areas e.g. during motor tasks and motor imagery. Kaplan (1979) noted a lack of appropriate spatial filtering would make detection of SMR difficult due to the stronger occipital alpha waves. 

(d) Network reactivation 

• Wilson & McNaughton (1994) demonstrated that neuronal activity patterns detected during a learning task before sleep are reactivated in the brain during sleep. Brain regions linked to memory lead to the theory that sleep is involved in memory consolidation. 
• Researchers demonstrated the pre-motor and visual cortex areas involved are most active during REM sleep after a sequential motor task, rather than during NREM sleep. 
• Similarly, the hippocampal areas involved in spatial learning tasks are reactivated in NREM sleep, rather than in REM sleep, which suggested the role of sleep in consolidation of memory types. However, the mechanisms behind the consolidation of memory types in sleep is poorly understood. 

(e) Hippocampal neocortical dialog 

Schematic representation of the 2-stage reciprocal cortical-hippocampus dialogue. Information is carried by high-frequency signals. During wake and SWS, the information receiver coordinates the timing of transmissions from the sender via propagated low-frequency signals.
Source: https://www.pnas.org/content/113/44/E6868

• Buzsáki (1996) proposed the hippocampal neocortical dialog, referred to as the structural interactions during SWS. 
• Sharp wave patterns (SWP) are detected from the hippocampus during SWS and neurons in that brain region burst in organised fashion during this stage of sleep. This neuronal activity is synchronised with state changes in the cortex (DOWN/UP state) and coordinated by slow oscillations in the cortex. 
• These observations led to the hypothesis that the hippocampal neocortical dialog is a mechanism for the transfer of information from the hippocampus to the cortex. Ferrera et al. (2012) thought this dialog is involved in memory consolidation. 

How is sleep regulated? 
Sleep regulation is defined as the control of organism's transition between sleep and wakefulness. Siegel (2002) highlighted the key questions of sleep regulation, which include identification of the brain regions involved in sleep onset and their mechanisms of action. Lu et al. (2006) observed sleep and wakefulness in humans and most animals tended to follow an electronic flip-flop model i.e. both states are stable, but the intermediate states are not. 

I. Sleep onset 
https://en.wikipedia.org/wiki/Sleep_onset
Sleep onset is the transition from wakefulness into sleep, which usually transmits into NREM sleep but under certain circumstances (e.g. narcolepsy) the transition from wakefulness directly to REM sleep is possible. 

• During the 1920s, scientists observed an obscure disorder that caused encephalitis and attacked the brain region involved in sleep regulation in European and North American subjects. 
• Psychiatrist and neurologist Constantin von Economo studied this disorder and identified a key component in the sleep-wake regulation. Moreover, he identified the pathways involved in the regulation of wakefulness and sleep onset were between the brainstem and the basal forebrain. However, his discoveries were not accepted until the 1980s when the pathways of sleep were supported by supplementary studies. 

- Neural circuit: 

• Von Economo noticed the lesions in the connection between the midbrain and the diencephalon led to prolonged sleepiness and hence the concept of an ascending arousal system. 
• The major ascending pathways is divided into 2 branches: (1) one ascending to the thalamus and activating the thalamic relay neurons, and (2) the other activating the lateral part of the hypothalamus and the basal forebrain, and throughout the cerebral cortex. This refers to the ascending reticular activating system (cf. reticular formation). 
• The cell group involved in the first pathway is an acetylcholine-producing cell group called pedunculopontine and laterodorsal segmental nucleus (PPT / LDT). Neurons in these regions are involved in bridging information in between the thalamus and the cerebral cortex. They are highly active both during wakefulness and REM sleep, and lowly active during NREM sleep. 
• The second pathway involves monoaminenergic neurons located in the locus coeruleus, dorsal and median raphe nuclei, ventral periaqueductal grey matter, and tuberomammillary nucleus. 
• The locus coeruleus releases noradrenaline; the dorsal and median raphe nuclei releases serotonin; the ventral periaqueductal grey matter releases dopamine; the tuberomammillary nucleus releases histamine. 
• All these brain regions projects onto the hypothalamic peptidergic neurons containing melanin-concentrated hormones or orexin, and the basal forebrain neurons containing GABA and acetylcholine. These neurons then project onto the cerebral cortex. 
• Studies observed lesions to the aforementioned regions of the brain lead to prolonged sleep or a coma. 

- Lesions: 

• Sallanon et al. (1989) discovered lesions in the preoptic area and anterior hypothalamus led to insomnia and lesions in the posterior hypothalamus led to sleepiness. 
• Saper et al. (2001) demonstrated the hypothalamic region called the ventrolateral preoptic nucleus releases GABA that inhibits the arousal system during sleep onset. 

- Direct mechanism: 

• Sleep onset is triggered by sleep-promoting neurons, located in the ventrolateral preoptic nucleus (VLPO). Researchers thought these neurons transmitted GABA type A and galanin to arousal-promoting neurons such as histaminergic, serotonergic, orexinergic, noradrenergic, and cholinergic neurons. 
• Carlson (2013) found the levels of acetylcholine, noradrenaline, serotonin, and histamine decrease with the onset of sleep, since they are all wakefulness-promoting neurotransmitters. This lead to the hypothesis that the activation of sleep-promoting neurons inhibits the arousal-promoting neurons, inducing sleepiness. 
• Studies found that during the sleep-wake cycle, sleep-promoting neurons and the arousal-promoting neurons reciprocally project to each other. Furthermore, the concentration of GABA receptors increase in the arousal-promoting neurons during NREM sleep. Researchers theorised the increase of GABA receptors in the arousal-promoting neurons is another pathway of inducing sleep. 
• During the times of increased brain activity, such as during daytime, astrocytes supplies glycogen to be converted into energy for neurons, therefore prolonged wakefulness induces a decrease in glycogen levels in the brain. This increases the level of extracellular adenosine, which inhibits neural activity. Adenosine demonstrates to be a sleep-promoting nucleoside neuromodulator. 
• A large proportion neurons involved in sleep is in the ventrolateral preoptic area (vlPOA). Takahashi et al. (2009) observed vlPOA neurons are inactive until the person transitions from wakefulness to sleepiness. 
• Chou et al. (2002) found vlPOA neurons gets reciprocally inhibited from some brain regions involved in sleep, such as the tubermammillary nucleus, raphe nuclei, and locus coeruleus, hence they are inhibited by histamine, serotonin and noradrenaline. 
• Saper et al. (2011) explained this reciprocal inhibition is known as the flip-flop, which switches the activation of specific neurons from these brain regions from one state to another rapidly between wakefulness and sleepiness. 

II. Models of sleep regulation

• Sleep is regulated by 2 parallel mechanisms: homeostatic regulation and circadian regulation, controlled by the hypothalamus and suprachiasmatic nucleus (SCN), respectively. 
• Although the exact nature of sleep drive is unknown, homeostatic pressure accumulates during wakefulness to the point the person falls asleep. Despite adenosine being known to induce sleep, it doesn't the accumulation of homeostatic pressure. 
• In 1982, Borbély first proposed a two-process model, namely Process S (homeostatic) and Process C (circadian) respectively. He demonstrated slow wave density increases through the night and then peters out at the start of the day. 
• In 1993, Edgar et al. (1993) proposed a different model, namely the 'opponent process model'. It suggested that Process S and Process C opposed each other to induce sleep, contrary to Borbely's model. According to this model, the SCN facilitates wakefulness and opposes the homeostatic rhythm. In addition, the homeostatic rhythm is regulated via a complex multisynaptic pathway in the hypothalamus that can shut off the arousal system. 
• Saper et al. (2005) observed both effects combined generated a teetering effect of sleep and wakefulness. Birendra et al. (2011) thought both models were valid to some extend, while other theories suggested REM could inhibit NREM sleep. 

III. Thalamic regulation 

• A majority of brain activity in sleep is generated by the thalamus, indicating its role in SWS. Delta waves and slow oscillations are primarily produced by the thalamus and the cortex in SWS. However, sleep spindles are only produced by the thalamus. 
• McCormick & Bal (1997) proposed the thalamic pacemaker hypothesis these oscillations are produced by the thalamus but the synchronisation of a number of groups of thalamic neurons firing simultaneously depends on the thalamic interaction with the cortex. 
• Studies suggested the thalamus activates in sleep onset during the transition from tonic to phasic mode, hence acting as a mirror for both central and decentral elements and connecting remote regions of the cortex to coordinate their activity. 

IV. Ascending reticular activating system (ARAS)

• The ARAS comprises of a set of neural subsystems projecting from a number of thalamic nuclei, as well as dopaminergic, noradrenergic, serotonergic, histaminergic, cholinergic, and glutamatergic brain nuclei. 
• During wakefulness, the ARAS receives a range of non-specific sensory information and relays them to the cortex. Furthermore, it regulates the fight or flight responses, associated with the motor system. 
• During sleep onset, the ARAS acts via 2 pathways: (1) a cholinergic pathway projecting to the cortex via the thalamus and (2) a collection of serotonergic pathways projecting to the cortex via the hypothalamus. 
• During NREM sleep, the ARAS is inhibited by GABAergic neurons in the ventrolateral preoptic area and parafacial zone, as well as other sleep-promoting neurons in distinct brain regions. 

What are the functions of sleep? 
The function of sleep is the least understood in sleep research. Researchers theorised sleep evolved to fulfil a primordial function and acquired multiple functions over time. 
Many hypotheses have been put forward to explain the function of sleep, which reflects the incomplete understanding of sleep. 

I. Body restoration 

• Sleep is known to associate with wound healing and immune system function. Opp (2009) stated the "sleep deprivation impairs immune function and immune challenge alters sleep", which suggests sleep increases white blood cell counts. Peres (2014) found sleep deprivation in mice increased cancer growth and weakened the immune system's ability to control cancers. 
• Jenni et al. (2007) found a lack of significance between sleep debt and somatic growth in children. On the other hand, Van Cauter et al. (2000) discovered a correlation between SWS and GH secretion in men. 
• A 2013 reported a brain removes metabolic waste at a faster rate in sleep state compared to an awake state. During wakefulness, metabolism generates reactive oxygen species (ROS), which damages the DNA inside cells. When an organism sleeps, metabolic rate decreases and ROS generation also decreases, which facilitates restorative processes to remove ROS. 
• Siegel (2005) speculated that sleep promotes the synthesis of molecules (e.g. antioxidants) that help repair and protect the brain from ROS. Since the metabolic phase during is anabolic, hormones released during this phase include GH. 
• When an organism rests quiescently, they may conserve energy without shutting down their body, but it places them in a precarious situation. A sedentary awake animal can still preserve energy and is likely to survive predators. 
• This led to hypotheses that the purpose of sleep is restoration of synaptic strength. Cirelli & Tononi (2013) also suggested unnecessary connections are weakened to better promote learning and memory functions once more for the next day. This indicates the brain forgets some things learnt each day. 

II. Memory processing & Learning

https://en.wikipedia.org/wiki/Sleep_and_memory
https://en.wikipedia.org/wiki/Sleep_and_learning

• Plihal & Born (1997) suggested sleep increases the recall of previous learning and experiences, and its benefit is determined on the sleep stage and the type of memory. Rasch et al. (2007) observed declarative memory improves significantly during early sleep (mainly SWS) while procedural memory improves during late sleep (mainly REM sleep). 
• Born (2012) associated the functional role of SWS with replays of previously encoded neural patterns in the hippocampus that facilitate consolidation of long-term memories, particularly declarative memories. 
• Diekelman & Born (2010) proposed the active system consolidation hypothesis. This refers to repeated reactivations of newly encoded information in the hippocampus during slow waves in NREM sleep regulating the stabilisation and gradual integration of declarative memory with pre-existing knowledge networks cortically. 
• Researchers surmised the hippocampus stored information temporarily and in fast-learning rate, whereas the neocortex is associated with long-term storage and slow-learning rate. 
• Scientists also stated that the communication between the hippocampus and neocortex coincides with hippocampal sharp-wave ripples and and thalamo-cortical spindles. This synchrony drives the production of spindle-ripples, which may be essential for the formation of long-term memories. 
• Born (2012) stated memory reactivates during an awakened state, which serves to update the reactivated memory with newly encoded information. In addition, when memory reactivates during SWS, it indicates memory stabilisation. 
• Several studies used targeted memory reactivation (TMR) experiments to emphasise the importance of nocturnal reactivations for the development of long-term memories in neocortical networks. Furthermore, those reactivations highlight the possibility of increasing people's memory performance at declarative recall tasks. 
• Schreiner et al. (2018) observed nocturnal reactivation shared similarities with the neural oscillatory patterns as diurnal reactivation, which may be coordinated by theta activity. 
• Schreiner et al. (2015) correlated theta rhythms with outstanding performance in memory tasks during wakefulness. Furthermore, cued memory reactivations during sleep demonstrated that theta oscillations is prominent in subsequent recognition of cued stimuli compared to uncued stimuli. This implied memory traces and lexical integration being consolidated by cuing during sleep. Groch et al. (2017) alluded to the fact that the benefits of TMR for memory consolidation only occurred when the cued memories is associated with prior knowledge. 
• Studies found sleep deprivation doesn't significantly influence a person's recognition of faces, nonetheless does significantly impair temporal memory. Furthermore, sleep deprivation increased the prevalence of beliefs of being correct, especially if such beliefs contradicts reality. 
• Sleep deprivation also worsened subjects' performance on free recall of a list of nouns compared to subjects meeting the recommended amount of nightly sleep. This finding emphasises sleep's role in the development of declarative memory. Subsequent imaging studies detected low metabolic activity in the prefrontal cortex and temporal and parietal lobes during the temporal learning and verbal learning tasks in sleep deprived patients. 
• Neural networks during SWS significantly correlated with templates than during wakefulness or REM sleep. Moreover, after the learning process, post-SWS reverberations lasted for a further 48 hours, which was longer than the duration of novel object learning (1 hour), which suggested long-term potentiation. 

III. Preservation 

• Choi (2009) proposed the "preservation and protection" theory" based on the idea that sleep serves an adaptive function. It suggested the animal is protected during sleep in which being awake would place the individual at highest risk. However, it fails to expound the reasons behind the brain's disengagement from the external environment during sleep. 
• Some researchers thought that circadian regulation sufficiently explains the periods of activity and quiescence the organism adapts to, however the mysterious specialisations of sleep serve functions that are currently unknown.  

IV. Clearing waste from the brain 

• Studies found metabolic waste products, such as immunoglobulins, protein fragments or intact proteins such as beta-amyloid, is eliminated from the interstitium via a glymphatic system of lymph-like channels flowing through perivascular spaces and the astrocyte network of the brain during sleep. 
• This model implied hollow tubes between the blood and vessels and astrocytes function as a spillway to drain cerebrospinal fluid that carries waste periods out of the brain into systemic blood. 
• Nedergaard & Goldman (2016) indicated potential mechanisms that suggest sleep is a regulated maintenance period for brain immune functions and clearance of beta-amyloid, a risk factor of Alzheimer's Disease. 

V. Endocrine function 

• Melatonin is regarded as a circadian hormone, whose secretion increases at dim light and peaks during nocturnal sleep, and decreases with bright light shone on the eyes. Some organisms showed melatonin is released in the brain upon sleep, but human brains secretes melatonin independent of sleep and dependent on light levels. 
• Leproult et al. (2001) claimed cortisol and thyroid stimulating hormone (TSH) are circadian and diurnal hormones, independent of sleep. 
• Van Cauter et al. (1992) found other hormones such as growth hormone (GH) and prolactin are sleep-dependent, and are suppressed in the absence of sleep. GH is maximally increased during SWS, whereas prolactin is secreted early after sleep onset and increases through the night. 
• Kern et al. (1996) found cortisol is critical for metabolism and controls the ability to endure noxious stimuli, which increases as one wakes up and is in REM sleep. 
• Knutson et al. (2007) observed TSH increases during nocturnal sleep and decreases with prolonged periods of reduced sleep, but increases during total acute sleep deprivation. 
• Since hormones is associated with energy balance and metabolism, and sleep is associated with the timing and amplitude of their secretion, sleep may play a role in metabolism. 

VI. Renormalisation of the synaptic strength 

• Sleep may occur to weaken synaptic connections acquired over the course of the day that may not be important to optimal functioning. 
• Tonini (2011) thought this minimises biological resource demands, since the maintenance and potentiation of synaptic connections accounts for a significant portion of energy consumption by the brain and taxes other cellular mechanisms such as protein synthesis for new channels. 

VII. Behaviour change with sleep deprivation

• Sleep deprivation is a prevalent issue in modern societies due to occupational and domestic reasons such as round-the-clock service, security or media coverage, cross-time-zone projects etc.. 
• Dement's study on REM sleep deprivation found humans demonstrated more REM sleep than usual if they experienced REM sleep deprivation, a phenomenon he named "REM rebound". 
• Van Dongen et al. (2003) found a correlation between sleep deprivation and the increased probability of accidents and industrial accidents. Knutson et al. (2007) demonstrated metabolic activity decreases in brains that accumulated many hours of sleep debt. 
• Researchers found sleep derivation created attention deficits, which may be masked by alternate activities such as standing or walking, or caffeine consumption. 
• Several studies demonstrated sleep deprivation led to poor performances on cognitive tasks, especially those involving divergent functions or multitasking. 
• Researchers also found sleep deprivation led to changes in mood and emotion, increasing the tendency to rage, fear or depression. Chee & Chuah (2008) highlighted the variability of the effects from person to person under identical amount of sleep debt. 
• Nevertheless, the mechanisms for such phenomenon are still unknown and the neural pathways and cellular pathways of sleep debt requires further research. 

Discuss the ontogeny and phylogeny of sleep 

1. How did sleep evolve in the animal kingdom? 
2. How did sleep develop in humans? 
3. How do other animals sleep? 

https://en.wikipedia.org/wiki/Sleep_in_non-human_animals

(1) Sleep evolution 

Evolution of REM sleep and genomic imprinting. The timeline (presented as millions of years ago, MYA) on the horizontal axis maps geological and glacial periods and the evolutionary divergence that occurred among reptiles, birds, and mammals (monotremes, eutherians, and marsupials). The red line delineates the presence of genomic imprinting in therians. A description of REM sleep according to different species is annotated on the right. 
Sources: https://www.researchgate.net/figure/Evolution-of-REM-sleep-and-genomic-imprinting-The-timeline-expressed-as-millions-of_fig2_303552062

• Studies theorised different types of sleep patterns evolved due to a number of selective pressures, such as body size, relative metabolic rate, predation, type and location of food sources, and immune function. 
• Sleep (esp. deep SWS and REM) may have evolved to gain an advantage over the predation risk it involves. Researchers noted how different organisms balanced this risk by developing partial sleep mechanisms or living in protective habitats. This hinted not only to the developmental aspects and mechanisms, but also to the adaptive justifications for sleep. 
• Sufficient sleep information is known only for 2 phyla of animals: chordata and arthropoda. The question sleep scientists attempt to answer is whether sleep evolved only once or numerous times. 
• It's known humans demonstrate both SWS and REM sleep, in both phases both eyes are closed and both hemispheres of the brain are involved. Studies have found other mammals such as echidnas, therians and odontocedes (e.g. dolphins and proposes) also demonstrated REM sleep. 
• Aquatic animals were found to perform unihemispherical slow wave sleep (USWS), which means one brain hemisphere is less electrically active than the other brain hemisphere. Researchers suggested USWS allows the animal to minimise predator risk and sleep while swimming in water. 
• Roth et al. (2006) found birds similar sleep patterns to mammals, with similar features of SWS and REM sleep, including closure of both eyes, relaxed muscle tone, etc. If the predation risk is high in the surrounding environment, some bird species can sleep with 1 eye open, which leads to the hypothesis that birds may sleep in flight during migration. However, more research is needed to explain whether birds sleep during flight or whether there are other mechanisms ensuring birds remain healthy during long flights in the absence of sleep. 
• Stickgold (2009) found reptile species don't demonstrate REM sleep. 
• In some invertebrates, such as fruit flies (Drosophila) and honeybees, scientists have discovered several sleep-related features similarly seen in mammals such as decreased reaction to sensory input, lack of motor response in the form of antennal immobility, etc. 
• Current findings suggests sleep evolved independently in mammals and birds, and reptiles. However, more research is required to understand the EEG features of sleep and its role in long-term plasticity. 
• Tsoukalas (2012) implied REM sleep is an evolutionary modification of a well-known mechanism called the tonic immobility reflex. Also known as animal hypnosis or death feigning, this reflex functions as the last line of defence against an attacking predator, which involves total immobilisation of the animal. 
• This action is commonly known as "playing possum" or "playing dead". The neurophysiology and and phenomenology of this reaction demonstrates similarities to REM sleep, which reveals a deep evolutionary association. Vitelli (2013) discovered the tonic immobility reflex and REM sleep exhibit brainstem control, paralysis, sympathetic activation, and thermoregulatory changes. 

(2) Sleep development and ageing 

• The ontogeny of sleep studies sleep patterns across different age groups of a species, particularly during development and ageing. 
• Among mammals, infants sleep the longest around 16 hours a day (8 hours REM sleep and 8 hours NREM sleep) on average. Gertner et al. (2002) correlated the degree of precociality of the child to the variance of time spent in each sleep mode in infants during the first few weeks of development. 
• Van Cauter et al. (2000) noticed a significant reduction in the proportion of hours spent in REM sleep within the few months of postnatal development. When an child matures into an adult, they spend about 6-7 hours in NREM sleep and about 1 hour in REM sleep.
• Ibuka (1984) found this relationship is demonstrated not only in humans, but also animals dependent on their parents for food. Gerter et al. (2002) hypothesised that REM sleep facilitates early brain development in infants. 
• Ohayan et al. (2004) thought the decrease in total sleep time from childhood to adolescence correlated with environmental factors rather than biological factors such as homeostatic regulation mechanisms. 
• Furthermore, Ohayan et al. (2004) demonstrated both the sleep latency and the time spent in NREM1 and 2 increases with ageing, and the time spent in REM and SWS decreases with ageing. 
• Several studies concluded that these findings relate to brain atrophy, cognitive impairment and neurodegenerative disorders in old age. Researchers linked the decreases in REM sleep and SWS in older humans with declines in declarative memory consolidation in midlife and atrophy in the medial prefrontal cortex (mPFC). 
• Zhong & Rogers (2011) highlighted sleep disturbances, such as excessive daytime sleepiness and nighttime insomnia, as a risk factor of progressive functional impairment in Alzheimer's Disease (AD) or Parkinson's Disease (PD). 
• Weitzman et al. (1981) observed many older adults spend time awake in bed after sleep onset, which demonstrated their inability to fall asleep and decrease in sleep efficiency. 

(3) Sleep in non-human animals 
https://jeb.biologists.org/content/221/11/jeb159533
i. Invertebrates 

• Unicellular organisms don't necessarily sleep, but some species are observed to have circadian rhythms. 
• Anafi et al. (2019) observed sleep states in jellyfish, such as Cassiopea, didn't need a brain or central nervous system. 
• The nematode C. elegans demonstrates the need for sleep, with a fatigue phase occurring in short periods preceding each moult, which is evidence for sleep being primitively linked to developmental processes and changes in the neural system. 
• Although insects experience circadian rhythm of activity and passivity, they don't demonstrate homeostatic sleep, especially REM sleep. 
• However, fruit flies are found to sleep, and sleep disturbances lead to cognitive disabilities. Similar findings are observed in arthropods, cockroaches, crayfish and honeybees. 

ii. Fish 

• Typical fish reveal periods of inactivity but doesn't demonstrate significant reactions to sleep deprivation. Kavanau (1998) found fish species living in shoals or swimming continuously (e.g. due to the need for ram ventilation of the gills) didn't sleep. 
• Nelson (1970) found the following fish species that sleep: zebrafish, tilapia, tench, brown bullhead, and swell shark. They become motionless and unresponsive at night (in the case of a swell shark, at day). 
• Tauber (1974) found the Spanish hogfish and blue-headed wrasse become unresponsive when sleeping. 
• Weber (1961) observed approximately 200 species of European public aquaria exhibited apparent sleep. 
• Reebs (2002) noted these sleep patterns are easily disrupted and could disappear during periods of migration, spawning, and parental care. 

iii. Reptiles and amphibians 

• Like mammalian sleep, reptiles have quiescent periods, and a decrease in neural activity during sleep. 
• Nicolau et al. (2000) highlighted the differing EEG pattern in sleeping reptiles compared to sleeping mammals. Reptiles require more sleep following sleep deprivation, and powerful stimuli is required to awaken them. 
• Shien-Idelson et al. (2016) found REM- and NREM-like sleep stages in the Australian dragon Pogona vitticeps. 

iv. Birds 

• Kavanau (2002) proposed the concept that sleep in higher animals with its split into REM and NREM sleep may have evolved together with warm-bloodedness. Martinez-Gonzalez et al. (2008) suggested birds experience deeper or more intense SWS to compensate for sleep loss. 
• Roth et al. (2006) implied that birds sleeping in environments with exposure to predators have less deep sleep than birds sleeping in more protected environments, which may account for the variations in sleep amounts. 
• Rattenborg et al. (2001) thought some birds experience unihemispheric sleep-wave sleep (USWS), which involves the ipsilateral hemisphere sleeping and the contralateral eye closing. 
• Rattenborg et al. (1999) found ducks sleeping near the perimeter of the flock likely detects predator attacks due to having more USWS than ducks sleeping in the middle of the flock. 

v. Mammals 

• Mammals demonstrate diversity in sleep phenomena, such as periods of alternating of NREM and REM sleep. 
• Horses and other herbivore ungulates (e.g. Giraffes) sleep while standing, but lie down for REM sleep. Bats are known to sleep upside-down in caves and on tree branches. Male armadillos become erect during NREM sleep, but on the contrary for rats.  
• Early mammals demonstrated more polyphasic sleep, which involved higher daily sleep quotas and shorter sleep cycles compared to monophasic sleep. 
• Capellini et al. (2008) suggested monophasic sleep was naturally selected to attain larger mammalian body sizes and therefore lower BMR. 
• Daan et al. (1991) observed that mammals sleep even during the hypometabolic state of hibernation, manifesting in a net floss of energy as the animal transitions from hypothermia and euthermia in order to sleep. 
• Nocturnal animals have higher body temperatures, higher brain activity, increased serotonin levels, and decreased cortisol levels during the night, and vice versa for diurnal animals. 
• Etienne (2007) found nocturnal and diurnal animals both demonstrated increased electrical activity in the suprachiasmatic nucleus (SCN), and corresponding secretion of meltonin from the pineal gland, at night. Fred et al. (1999) added nocturnal animals have higher melatonin at night similar to diurnal animals, which tend to stay awake at night. 
• Van Cauter & Spiegel (1999) found cortisol levels increase throughout the night in diurnal animals, peak in the wakefulness hours, and decrease during the day. 

- Average Duration (out of 24 hours) 

• Horses = 2 hours 
• Elephants = 3+ hours 
• Cows = 4 hours 
• Giraffes = 4.5 hours 
• Humans = 8 hours 
• Rabbits = 8.4 hours 
• Chimpanzees = 9.7 hours 
• Red foxes = 9.8 hours 
• Dogs = 10.1 hours 
• House mice = 12.5 hours 
• Cats = 12.5 hours 
• Lions = 13.5 hours 
• Platypus = 14 hours 
• Rats = 14 hours 
• Chipmunks = 15 hours 
• Giant armadillos = 18.1 hours 
• Little brown bats = 19.9 hours 
• Koalas = 21 hours 

- Rodents 

• Datta (2000) conducted rat experiments and observed RME sleep improved memory performance. Kudrimoti et al. (1999) observed patterns of hippocampal place cells are reactivated during SWS after a spatial explorative activity. 
• Studies found rats lose weight and decrease their body temperature when deprived of sleep. Furthermore, they develop skin lesions, hyperphagia, lose body mass, hinder the healing process, experience hyperthermia, and eventually, fatal sepsis. In addition, sleep-deprived rats had a 20% decrease in white blood cell count compared to control rats. 
• Jeres (2012) observed sleep-deprived mice demonstrated more cancer growth and diminished ability of the immune system to mediate cancer tumours. They discovered increased levels of M2 tumour-associated macrophages and TLR4 molecules in sleep-deprived mice, which may be the mechanism for increased susceptibility to cancer growth. It's known M2 macrophages suppress the immune system and promote tumour grwoth, and TLR4 molecules signal activation of the immune system. 

- Monotremes 

• Initially, monotremes (egg-laying mammals) were thought to not sleep until after a clade of monotremes branched from the main mammalian evolutionary line to become a separate group. 
• Siegel et al. (1996) observed the brainstem EEG firing patterns in monotremes were similar to firing patterns during REM sleep of higher mammals. 
• Siegel et al. (1999) noted the platypus spent more time in REM sleep than any other animal. However, the platypods' forebrains didn't exhibit REM electrical activation, indicating the lack of dreaming. 
• Holland (2011) calculated the average sleep time of the platypus in a 24-hour period is around 14 hours, possibly due to their high-calorie crustacean diet. 

- Aquatic mammals 

• If marine mammalian species fell into deep sleep, they may suffocate and drown, or be vulnerable to predators. Therefore, dolphins, whales, and pinnipeds (seals) perform unihemispheric sleep while swimming. 
• Eared seals, sleep unihemispherically while earless seals sleep bihemispherically under water, hanging on the water surface or on land. 
• Although whales don't experience REM sleep, a species of dolphin called the pilot whale exhibits REM sleep. Nevertheless, researchers suggested the reason why marine animals don't REM sleep is the fact that REM sleep causes muscular atony. 
• Siegel (2005) defined muscular atony as a functional paralysis of skeletal muscles that can inhibit regular breathing. 
• Sekiguchi et al. (2006) found toothed cetaceans exhibited USWS while in captivity. 
• Miller et al. (2008) discovered sperm whales sleep vertically beneath the surface in passive shallow 'drift-dives', during daytime, meaning whales are unresponsive to passing vessels unless physically contacting them. 

- Hibernation: 

• Hibernating animals transition into a state of torpor, which is different from regular sleep. Daan et al. (1991) stated these animals terminate their hibernation during the winter to sleep. Moreover, they wake up from hibernation to transition into rebound sleep due to a lack of sleep during the hibernation period. Although these animals recover fully and conserve energy during hibernation, more research is required to understand the need for hibernating animals to sleep.  

What is unihemispheric slow-wave sleep (USWS)? 
https://en.wikipedia.org/wiki/Unihemispheric_slow-wave_sleep

Examples of unihemispheric slow-wave sleep in the bottle-nosed dolphin (Tursiops truncatus). The EEG was recorded from the anterior, medial, and posterior neocortex. Note the high-amplitude, low-frequency activity indicative of slow-wave sleep (blue) in only the left (a) or the right (b) hemisphere concurrent with low-amplitude, high-frequency activity indicative of wakefulness (red) in the other hemisphere.
Source: Mukhameotv, LM, Supin AY, and Polyakova IG (1977) Interhemispheric asymmetry of electroencephalographic sleep patterns in dolphins. Brain Resarch 134: 581-584
https://www.researchgate.net/figure/Examples-of-unihemispheric-slow-wave-sleep-in-the-bottle-nosed-dolphin-Tursiops_fig2_285796850

• Also known as asymmetric SWS, USWS involves one brain hemisphere resting while the other brain hemisphere remains active and alert. In biological terms, one brain hemisphere is in NREM sleep, and the eye corresponding to this hemisphere is closed while the other eye remains open. Thus, the other brain hemisphere is still in wakefulness state. 

Physiology of USWS: 

• Acetylcholine is associated with hemispheric activation in northern fur seals, since it is released laterally to the hemisphere exhibiting a wakefulness EEG pattern. Therefore, the hemisphere exhibiting a SWS EEG pattern has minimal amounts of acetylcholine. 
• Domestic chicks and other bird species that demonstrate USWS are found to have the contralateral eye opened relative to the "wakefulness" hemisphere, and the ipsilateral eye closed relative to the SWS hemisphere. This helps birds look out for any predators within its vicinity mid-flight. 
• A 1990 study found brain temperature decreases when at least one hemisphere exhibits a SWS EEG. This associates with mechanisms responsible for thermoregulation and energy conservation as attentiveness of USWS maintains. 

Anatomical variations of USWS: 

• Researchers pointed out that USWS needs hemispheric segregation to isolate the cerebral hemispheres. The corpus callosum is a mammalian brain structure responsible for interhemispheric communication. 
• Cetaceans and birds were found to have smaller corpus callosum than other mammals. Rattenbourg et al. (2000) contradicted this theory by observing sagittal transections of the corpus callosum led to strictly bihemispheric sleep. More research is required to directly explain the existence of USWS in the brain. 
• Studies discovered sagittal transections of subcortical regions, including the lower brainstem, caused asynchronous SWS in non-USWS-exhibiting animals. 
• Rattenbourg et al. (2000) discovered USWS-exhibiting mammals had larger posterior commissure and elevated decussation of ascending fibres from the locus coeruleus in the brainstem. This associates the noradrenergic diffuse modulatory system in the locus coeruleus, which mediates arousal, attention, and sleep-wake cycles. 
• Studies observed higher noradrenergic release in the awaken hemisphere and lower release in the SWS hemisphere. This leads to differences in brain temperature in each hemisphere, which may associate in shifting between the SWS and awakened state. 
• Complete decussation of the nerves at the optic chiasm may lead to the open eye strictly activating the contralateral hemisphere. However, Rattenbourg et al. argues this theory alone doesn't explain why blindness theoretically preventes USWS if only retinal nerve stimuli were present. 

What are the benefits of USWS? 

1. Adaptation to high-risk predation 

• Most bird species exhibit USWS to detect approaching predators, which is achieved by maintaining visual vigilance and keeping one eye open. This use of USWS by avian species is directly proportional to the risk of predation. 
• In response, both cetaceans and birds evolved behavioural mechanisms to increase the likelihood of evading predators. 
• Rattanborg et al. (1999) proposed the "group edge effect", which involves flocks of birds working together to detect and evade predators. Birds at the edge of the flock spend more time in USWS, since they are the most vulnerable to predation, hence need more vigilance compared to birds at the centre of the flock. Furthermore, birds on the left side of the flock would have their left hemisphere in awakened state and right hemisphere in SWS, to ensure their left eye is open, and vice versa for birds on the right side of  the flock. 

2. Surfacing for air and pod cohesion 

• Rattanborg et al. (2000) found USWS gave aquatic mammals, such as dolphins (e.g. bottlenose dolphin) and seals, the capability to simultaneously sleep and swim to the surface to breathe. 
• Rattenborg et al. (2006) observed the pods of Pacific white-sided dolphins demonstrated the reversed version of the bird's "group edge effect". He suggested this species helps maintain pod cohesion and formation while sustaining USWS. 

3. Rest during long bird flights 

• During migration, birds undergo USWS to both sleep and visually navigate mid-flight, which theoretically saves time wasted on frequent rest stops throughout the journey. However more research is required to investigate the inter-hemispheric EEG asymmetry in idle birds compared to flying birds. 

Which animal species exhibit USWS? 

1. Cetaceans 

- Amazon river dolphin (Inia geoffrensis) 

- Beluga whale (Delphinapterus leucus) 

- Bottlenose dolphin (Tursiops truncates) 

- Pacific white-sided dolphin (Lagenorhynchus obliquidens) 

- Pilot whale (Globicephela scammoni) 

- Porpoise (Phocoena phocoena) 

2. Pinnipeds 

- Northern fur seal (Callorhinus ursinus) 

- Southern sea lion (Otari bryonia) 

- Stellar sea lion (Eumetopias jubatus) 

3. Sirenia 

- Amazonian manatee (Trichechus inunguis)

4. Birds 

- Common swift (Apus apus) 

- Common blackbird (Turdus merula) 

- Domestic chicken (Gallus gallus domesticus) 

- Glaucous-winged gull (Larus glaucescens) 

- Japanese quail (Coturnix japonica) 

- Mallard (Anas platyrhynchos) 

- Northern bobwhite (Colinus virginianus) 

- Orange-fronted parakeet (Aratinga canicularis) 

- Peregrine falcon (Falco peregrinus) 

- White-crowned sparrow (Zonotrichia leucophrys gambelli) 

How much sleep is ideal for a human? 

• The amount of sleep every human requires varies according to age and wellbeing. The amount of sleep deemed adequate is if it alleviates daytime fatigue or dysfunction.
• Lauderdale et al. (2008) found a moderate correlation between self-reported sleep and  actual sleep time as measured by actigraphy. 
• A 2010 study found subjects afflicted with sleep state misperception typically underreport the amount of hours slept (i.e. reporting 4 hours sleep despite actual 8 hours sleep). 
• Several studies concluded that 6-7 hours of sleep every night associates with longevity and cardiac health in humans, however many underlying factors behind this causality warrants further investigation. 

i. Children 

• When infants turn 2 years old, the size of their brains are about 90% of an adult-sized brain. It's known a considerable proportion of an infant's brain growth occurs during this period of life with the highest rate of sleep, which affect their ability to perform on cognitive tasks. 
• Several studies found children sleeping through the night and little night waking episodes demonstrated superior cognitive achievements and greater temperaments than other children. 
• Hupbach et al. (2009) noted infants who slept within 4 hours of learning a language demonstrated greater memory of the language rules, compared to infants who stayed awake longer. 
• Berrier et al. (2010) suggested a direct relationship between infants' vocabulary and the amount of sleep, with 12 month old infants sleeping longer attaining superior vocabulary at 26 months old.

ii. Recommendations 

Hours of sleep required for each age group 

It is evident sleep is an important behaviour of the animal kingdom that ensures every organism's survival. But what happens if organisms, such as humans, don't achieve adequate sleep? I'll answer that question in detail in another post.