sleep wiki:
eep is the regular state of natural rest observed in all mammals, birds and fish. Sleep is not actually "unconsciousness," but rather, it is a natural state of rest characterized by a reduction in voluntary body movement and decreased awareness of the surroundings. Therefore, since consciousness is literally the awareness of the surroundings, being asleep is just an altered state of consciousness, as opposed to being unconscious. 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.
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