Sleep is a temporary, reversible state of unconsciousness from which the subject can be aroused by adequate sensory stimuli.
REM sleep occupies about 25% of total sleep, NREM sleep about 75%, in a normal adult.
Functions of Sleep
Sleep restores physical and mental strength, consolidates learning and memory, and promotes growth via increased growth-hormone secretion (less in REM).
Sleep also enhances the ability to think and concentrate.
Sleep deprivation in infants and children, who spend more time asleep to support growth, causes growth deficiency.
Dreams are produced during sleep; their usefulness depends on the ability to remember and interpret them.