Adults cycle through sleep in various stages of rapid eye movement (REM) and non-rapid eye movement (NREM), with NREM sleep divided into four stages.
A person's sleep time (approximately hours for adults) can be thought of as 2 halves. The first half for a majority of people consists mostly of Stages 2 and 3.
NREM sleep can be broken down into three distinct stages: N1, N2, and N3. In the In healthy adults, sleep typically begins with NREM sleep.
Find out what happens during the various stages of sleep with this guide to sleep cycles, including An average adult has five to six REM cycles each night.