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In animals, sleep is a naturally recurring state characterized by altered consciousness, relatively inhibited sensory activity, and inhibition of nearly all voluntary muscles. It is distinguished from wakefulness by a decreased ability to react to stimuli, and it is more easily reversible than being in hibernation or a coma.
During sleep, most systems in an animal are in a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems etc. It is observed in mammals, birds, reptiles, amphibians and fish, and (in some form) in insects and even simpler animals such as nematodes (see the related article Sleep (non-human)), suggesting that sleep is universal in the animal kingdom.
The purposes and mechanisms of sleep are only partially clear and the subject of substantial ongoing research. Sleep is sometimes thought to help conserve energy, though this theory is not fully adequate as it only decreases metabolism by about 5–10%. Additionally it is observed that mammals require sleep even during the hypometabolic state of hibernation, in which circumstance it is actually a net loss of energy as the animal returns from hypothermia to euthermia in order to sleep.
Humans may suffer from a number of sleep disorders. These include dyssomnias (such as; insomnia, hypersomnia, and sleep apnea) and parasomnias (such as sleepwalking and REM behavior disorder; and the circadian rhythm sleep disorders).
NREM stage 1: This is a stage between sleep and wakefulness. The muscles are active, and the eyes roll slowly, opening and closing moderately.
NREM stage 2: In this stage, theta activity is observed and sleepers become gradually harder to awaken; the alpha waves of the previous stage are interrupted by abrupt activity called sleep spindles and K-complexes.
NREM stage 3: Formerly divided into stages 3 and 4, this stage is called slow-wave sleep (SWS). SWS is initiated in the preoptic area and consists of delta activity, high amplitude waves at less than 3.5 Hz. The sleeper is less responsive to the environment; many environmental stimuli no longer produce any reactions.
REM: The sleeper now enters rapid eye movement (REM) where most muscles are paralyzed. REM sleep is turned on by acetylcholine secretion and is inhibited by neurons that secrete serotonin. This level is also referred to as paradoxical sleep because the sleeper, although exhibiting EEG waves similar to a waking state, is harder to arouse than at any other sleep stage. Vital signs indicate arousal and oxygen consumption by the brain is higher than when the sleeper is awake. An adult reaches REM approximately every 90 minutes, with the latter half of sleep being more dominated by this stage. REM sleep occurs as a person returns to stage 1 from a deep sleep. The function of REM sleep is uncertain but a lack of it will impair the ability to learn complex tasks. One approach to understanding the role of sleep is to study the deprivation of it. During this period, the EEG pattern returns to high frequency waves which look similar to the waves produced while the person is awake
30 seconds of deep (stage N3) sleep.
A screenshot of a PSG of a person in REM sleep. Eye movements highlighted by red box.
Sleep proceeds in cycles of REM and NREM, usually four or five of them per night, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.
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