In broad terms, the brain of someone who is sleeping cycles through two basic phases: rapid eye movement (REM) sleep and non-REM sleep. Non-REM sleep includes three different stages.1 A person cycles through REM sleep and non-REM sleep several times a night.
For more detailed information about what happens during sleep and phases of sleep, check out Your Guide to Healthy Sleep, from the National, Heart, Lung, and Blood Institute.
Sleep progresses in a cycle: from non-REM sleep stage 1 to non-REM sleep stage 2, to non-REM sleep stage 3, to REM sleep. Then the process starts over again with non-REM sleep stage 1.
The length of sleep stages changes during a given night’s sleep. For example, near the beginning of sleep, the body cycles through relatively short periods of REM sleep and long periods of deep sleep. As the night goes on, periods of REM sleep increase and those of deep sleep decrease. Near the end of a night of sleep, a person spends nearly all of their time in stages 1 and 2 and REM.3
Some of the characteristics of each phase include the following.