sleep

State of natural unconsciousness that occurs at regular intervals in most mammals and birds, though there is considerable variation in the amount of time spent sleeping. Sleep differs from hibernation in that it occurs daily rather than seasonally, and involves less drastic reductions in metabolism. The function of sleep is unclear. People deprived of sleep become irritable, uncoordinated, forgetful, hallucinatory, and even psychotic.

In humans, sleep is linked with hormone levels and specific brain electrical activity, including delta waves, quite different from the brain's waking activity. REM (rapid eye movement) phases, associated with dreams, occur at regular intervals during sleep, when the eyes move rapidly beneath closed lids.

Species that do not have distinct periods of sleep (most invertebrates and fishes, for example) have short intervals of reduced activity throughout a given 24-hour period. Extensive periods of sleep may have developed to make animals inconspicuous at times when they would be vulnerable to predators.

US researchers into REM sleep showed in 1998 that platypuses experience more REM (dream) sleep than any other animal. It had previously been thought that dreaming had evolved only in placental mammals.