The purposes and mechanisms of sleep are only partially clear and are the subject of intense research"

Source Wikipedia

Although extensive research in humans and animals has shown unequivocally that sleep is essential, the reasons why it's required are as yet unknown. Long term sleep deprivation not only severely impairs cognitive and motor skills, it's now been shown that it can cause physical damage to brain structure - and, in extreme cases, irreversible damage. ^{[ citation needed ]}

From an evolutionary point of view, laying unconscious for extended periods would presumably have made human and animal ancestors vulnerable to attacks, so the necessity of sleep must outweigh the considerable dangers it presents.

There are a large number of theories regarding the function and need for sleep - none has yet been generally accepted as a full explanation.

Hypotheses include:

• Memory consolidation - it's known that sleep deprivation can cause memory loss.

• Waste chemical removal - it's generally thought that transfers of cerebro-spinal fluid (which can transport amyloid) are more active during sleep. However, a 2024 research project from Imperial College, London found no evidence for this theory - finding instead that metabolite and toxin removal was actually slower during sleep. (ref.)

• Energy conservation - an awake brain uses more energy than a sleeping brain.

But, to quote William Dement, founder of Stanford University's Sleep Research Center,

As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy."

Source : (National Geographic Magazine, May 2010) (paywall)

A sleep phenomenon known as 'uni-hemispheric sleep' - i.e. the ability for just one half of the brain to engage in sleep (while the other half remains fully awake) - has been observed in numerous species, including birds, reptiles and some aquatic mammals (e.g. cetaceans, eared seals and manatees).

The relative absence of unihemispheric sleep in mammals suggests that a trade off exists between unihemispheric sleep and other adaptive brain functions occurring during sleep or wakefulness. Presumably, the benefits of sleeping unihemispherically only outweigh the costs under extreme circumstances such as sleeping at sea."

Source : Neurosci Biobehav Rev, 24(8):817-42.

(Note: The reasons why no land mammals can uni-hemispherically sleep is not known.)

The American Academy of Sleep Medicine (AASM) and Sleep Research Society (SRS) have developed a consensus recommendation for the amount of sleep needed to promote optimal health in adults, using a modified RAND Appropriateness Method process, and based on more than 4,000 previously published studies. They conclude that :

Sleeping less than 7 hours per night on a regular basis is associated with adverse health outcomes, including weight gain and obesity, diabetes, hypertension, heart disease and stroke, depression, and increased risk of death. Sleeping less than 7 hours per night is also associated with impaired immune function, increased pain, impaired performance, increased errors, and greater risk of accidents.
[…]
Sleeping more than 9 hours per night on a regular basis may be appropriate for young adults, individuals recovering from sleep debt, and individuals with illnesses. For others, it is uncertain whether sleeping more than 9 hours per night is associated with health risk."

See: Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society

There is currently no explanation as to why some mammals, like humans, appear to need about 8hrs of sleep in each 24hr period, and yet some other mammals routinely sleep 18hrs, whilst others seem to need only 3hrs or so. ^{[examples needed]}

Recent work on Hydra (and other animals without obvious brains) is suggesting that the urge to sleep might have originated in the very distant evolutionary past - long before brains had developed.

Researchers have noticed that molecules produced by muscles and some other tissues outside the nervous system can regulate sleep. Sleep affects metabolism pervasively in the body, suggesting that its influence is not exclusively neurological. And a body of work that’s been growing quietly but consistently for decades has shown that simple organisms with less and less brain spend significant time doing something that looks a lot like sleep.

See : Quanta Magazine

Note: Despite many decades of research, there is currently no scientific agreement about what constitutes 'a good night's sleep'. [Ref. BBC The Life Scientific audio podcast with professor Derk-Jan Dijk, director of the Sleep Research Centre, Univ. of Surrey, UK]

Also see :Infant Rockingplugin-autotooltip__plain plugin-autotooltip_bigInfant Rocking

"People tend to fall asleep when gently rocked or vibrated. Experimental studies have shown that rocking promotes sleep in humans and mice. However, the mechanisms underlying the phenomenon are not well understood."

Source : Cell Reports[Open Access]vol. 33, issue 9., Sleepwalkingplugin-autotooltip__plain plugin-autotooltip_bigSleepwalking

"Sleepwalking, also known as somnambulism or noctambulism, is a phenomenon of combined sleep and wakefulness. It is classified as a sleep disorder belonging to the parasomnia family. It occurs during slow wave sleep stage, in a state of low consciousness, with performance of activities that are usually performed during a state of full consciousness.", Insomnia (idiopathic)plugin-autotooltip__plain plugin-autotooltip_bigInsomnia (idiopathic)

"Up to one-third of the world’s population reports dissatisfaction with their sleep. Amongst the elderly and women, this percentage would be on the higher end. As high as 50% of older adults report sleep disturbances and insomnia. Using the DSM-5 criteria, up to 10% to 15% of adults would be classified as having short-term insomnia. Women are two times as likely to have insomnia than age-matched men." and Dreamingplugin-autotooltip__plain plugin-autotooltip_bigDreaming

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"The content and purpose of dreams are not yet understood, though they have been a topic of speculation and interest throughout recorded history.

Source : Wikipedia"

Not only is the function of dreaming unknown (if there is one), but there isn't as yet an agreed definition of what it is.

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