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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.