sleep stages
the stages of nocturnal sleep as distinguished by physiological measures, mainly scalp electroencephalography. Typically, a regular pattern of occipital alpha waves characteristic of a relaxed state becomes intermittent in Stage 1 sleep, which marked by drowsiness with upward-rolling eye movements. This progresses to Stage 2 sleep (light sleep), characterized by sleep spindles and K complexes. In Stage 3 sleep and Stage 4 sleep (deep sleep), delta waves predominate (see slow-wave sleep). These four stages comprise NREM sleep and are interspersed with periods of REM sleep, when most dreaming occurs. After a period of deep sleep, an individual may return to either light sleep or REM sleep, and the cycle can recur multiple times over a normal night.