sleep [%PREMIUM_LINK%] (slēp) Pronunciation Key |
A natural, reversible state of rest in most vertebrate animals, occurring at regular intervals and necessary for the maintenance of health. During sleep, the eyes usually close, the muscles relax, and responsiveness to external stimuli decreases. Growth and repair of the tissues of the body are thought to occur, and energy is conserved and stored. In humans and certain other animals, sleep occurs in five stages, the first four consisting of non-REM sleep and the last stage consisting of REM sleep. These stages constitute a sleep cycle that repeats itself about five times during a normal episode of sleep. Each cycle is longer that the one preceding it because the length of the REM stage increases with every cycle until waking occurs. Stage I is characterized by drowsiness, Stage II by light sleep, and Stages III and IV by deep sleep. Stages II and III repeat themselves before REM sleep (Stage V), which occurs about 90 minutes after the onset of sleep. During REM sleep, dreams occur, and memory is thought to be organized. In the stages of non-REM sleep, there are no dreams, and brain activity decreases while the body recovers from wakeful activity. The amount and periodicity of sleep in humans vary with age, with infants sleeping frequently for shorter periods, and mature adults sleeping for longer uninterrupted periods. See also non-REM sleep, REM sleep.