Human sleep divides into four stages of non-rapid eye movement sleep (NREM) and one of rapid eye movement sleep (REM), occurring in a ninety-minute cycle repeated around five times a night. Cycles gradually replace Stage 3 and 4 sleep with longer periods of alternating Stage 2 and 5 sleep: roughly half the final cycle is Stage 2, half Stage 5. The stages and the portions they represent of a normal night’s sleep are as follows:
Stage 1: Transitional sleep, 2–5 percent.
Stage 2: Baseline sleep, 45–60 percent: Brain waves follow “theta” rhythms.
Stages 3 and 4: Slow Wave sleep, 30–40 percent: Brain waves follow slower “delta” rhythms. This is the deepest and most restorative form of sleep.
Stage 5: REM sleep, 20–25 percent: Breathing, heart rate, and brain wave activity quicken. Dreams occur.