The period of non-REM sleep (NREM) is comprised of Stages 1–4 and lasts from 90 to 120 minutes, each stage lasting anywhere from 5 to 15 minutes. Surprisingly, however, Stages 2 and 3 repeat backwards before REM sleep is attained. So, a normal sleep cycle has this pattern: waking, stage 1, 2, 3, 4, 3, 2, REM.