People normally cycle through distinct stages of sleep every 90 to 120 minutes during the night: three stages of nonrapid eye movement sleep (N) and one stage of rapid eye movement (REM) sleep.

Relatively little time is spent in stage N1 (shallow) sleep. The greatest time is spent in stage N2 sleep. Deep sleep (stage N3) occurs mostly during the first half of the night. More time is spent in REM sleep as the night progresses. Brief awakenings (called stage W) occur throughout the night, but the sleeper is typically unaware of most of them.