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Abstract
Normal human sleep is comprised of two distinct states known as non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep is subdivided into four stages (stage 1, stage 2, stage 3, and stage 4). Stages 3 and 4 are collectively referred to as slow-wave sleep (SWS). REM sleep may be further subdivided into two stages: phasic and tonic. The NREM–REM sleep cycle occurs about every 90 min and approximately four to six cycles occur per major sleep episode. The early cycles are dominated by SWS and the later cycles are dominated by REM sleep. The first episode of REM sleep may last only a few minutes and subsequent REM episodes progressively lengthen in duration during the course of the major sleep period. In summary, SWS is prominent in the first third of the night and REM sleep is prominent in the last third of the night.