Hypnogram of a typical healthy adult. A hypnogram is a graph which shows the different stages of sleep: rapid eye movement sleep (REM) coloured dark blue, and non-rapid eye movement sleep (NREM). REM is associated with dreaming, and NREM is divided into stages N1, N2 and N3, when deep, slow-wave sleep occurs. Cycles of REM and non-REM occur, each cycle lasting about 90 minutes. A normal healthy adult requires 7?? hours of sleep per night, with 4-6 cycles. The number of hours of sleep varies according to age, health, emotional state, and environmental factors. Healthy adults typically spend 20??5% of their sleep in REM sleep.