sleep cycle

Sleep cycles alternate between non-REM and REM sleep, repeating roughly every 90 minutes. A typical cycle moves from light non-REM sleep into deeper non-REM sleep, then back toward lighter sleep and a shorter REM period. The exact cycle length varies between people, so 90 minutes is an average—not a reliable alarm-setting rule. 1

Across the night, the mix changes: early cycles contain more deep non-REM sleep, while later cycles contain more REM sleep and less deep sleep. 2