sleep [AS, slaepan, to sleep] , a state marked by reduced consciousness, diminished activity of the skeletal muscles, and depressed metabolism. People normally experience sleep in patterns that follow four observable, progressive stages. A device such as an encephalograph is used to record the recurrent pattern of brain waves during the stages. During stage 1 the brain waves are of the theta type, followed in stage 2 by the appearance of distinctive sleep spindles; during stages 3 and 4 the theta waves are replaced by delta waves. These four stages represent three fourths of a period of typical sleep and collectively are called nonrapid eye movement sleep. The remaining time is usually occupied with rapid eye movement (REM) sleep, which can be detected with electrodes placed on the skin around the eyes so that tiny electric discharges from contractions of the eye muscles are transmitted to recording equipment. The REM sleep periods, lasting from a few minutes to half an hour, alternate with the NREM periods. Dreaming occurs during REM time. Individual sleep patterns normally change throughout life because daily requirements for sleep gradually diminish from as much as 20 hours a day in infancy to as little as 6 hours a day in old age. Infants tend to begin a sleep period with REM sleep, whereas REM activity usually follows the four stages of NREM sleep in adults.
