Sleep is the regular state of natural unconsciousness observed in



all mammals, birds, and fish. It is heavily influenced by circadian

rhythms and by hormonal and environmental factors as well. Sleep

appears to perform a restorative function for the brain and body,

as evidenced by the myriad symptoms that result when an

individual is deprived of sleep.

The function of sleep in health and in disease is being increasingly

studied in specialized sleep laboratories throughout the world.

Not only insomnia, but also more recently elucidated sleep

disorders such as sleep apnea and narcolepsy are evaluated in

such facilities.

Before advances in the fields of neurology, neuroscience,

electronics, and genetics were made, scientists studied the

behavioral characteristics of sleep, such as its pattern, depth, and

varying frequency. In more recent times, the electrical impulses

generated by the brain are recorded using a device called an

electroencephalograph (EEG), and individual genes relating to

sleep-related brain function, such as the circadian rhythm, are

isolated. Molecular biology, medical science and epidemiology all

play an important role in modern studies of sleep.

Sleep is often defined using specific criteria relating to EEG data.

All mammals and birds fulfill the criteria for sleep based on EEG

recordings. In animals where EEG data is not readily available, or

their small size precludes recording EEG, behavioral and gene

specific data are utilized for sleep studies.

Sleep regulation

The cycle of sleep and wakefulness is regulated by the brain stem,

external stimuli, and by various hormones produced by the

hypothalamus. Certain neurohormones and neurotransmitters are

highly correlated with sleep and wake states. For example,

melatonin levels are highest during the night, and this hormone

appears to promote sleep. Adenosine, a nucleoside involved in

generating energy for biochemical processes, gradually

accumulates in the human brain during wakefulness but decreases

during sleep. Researchers believe that its accumulation during the

day encourages sleep. The stimulant properties of caffeine are

attributed to its negating the effects of adenosine.

The suprachiasmatic nucleus (SCN) of the hypothalamus plays an

important role in the regulation of circadian rhythms. The SCN is

influenced by external light and generates its own rhythm in

isolation. In the presence of light, it sends messages to the pineal

gland that instruct it to cease secreting melanin.

Thus, three processes, each influenced by hormonal, neurological,

and environmental factors, underlie sleep regulation:

¡¤A homeostatic process determined by prior sleep and

wakefulness, determining "sleep need".

¡¤A circadian process determining periods of high and low

sleep propensity, and high and low REM sleep propensity.

¡¤An ultradian process.

The interrelationships and relative importance of each process and

system remain uncertain.

Memory is highly dependent on sleep. REM sleep appears to help

with the consolidation of spatial and procedural memory, while

SWS helps with the consolidation of declarative memories. When

experimental subjects are given academic material to learn,

especially if it involves organized, systematic thought, their

retention is markedly increased after a night's sleep. Mere wrote

memorization is retained similarly well without an intervening

period of sleep.

REM sleep (or Active Sleep) seems to be particularly important to

the developing organism. Studies investigating the effects of

Active Sleep deprivation have shown that deprivation early in life

can result in behavioral problems, permanent sleep disruption,

decreased brain mass (Mirmiran et al. 1983), and an abnormal

amount of neuronal cell death (Morrissey, Duntley & Anch, 2004).

According to the Ontogenetic Hypothesis of REM sleep, the activity

occurring during neonatal REM sleep is necessary for proper central

nervous system development (Marks et al. 1995).

Given sleep's heterogeneous nature, it is difficult to describe a

single "function" of sleep. Based on current knowledge, it is

apparent that it has many functions.