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: