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Sleep - which is defined behaviorally by the suspension of normal consciousness and electrophysiologically by specific brain wave criteria - consumes fully a third of our lives.

Sleep occurs in all mammals, and probably all vertebrates. We crave sleep when deprived of it, and, to judge from animal studies, continued sleep deprivation can ultimately be fatal. Surprisingly, however, this peculiar state is not the result of a simple diminution of brain activity; rather, sleep is a series of precisely controlled brain states, and in some of these the brain is as active as it is when we are awake. The sequence of sleep states is governed by a group of brainstem nuclei that project widely throughout the rest of the brain and modulate overall levels of brain activity. Despite many advances, major aspects of sleep are still incompletely understood. The reason for high levels of brain activity during some phases of sleep, the significance of dreaming, and the basis of the restorative effect of sleep are all tantalizing puzzles that continue to motivate much ongoing research.

Teksten er hentet fra boken "Neuroscience", Purves et al., 1997. Sinauer forlag.