How Sleep Works Sleep Phases and Stages
When you sleep, you cycle through two phases of sleep: rapid eye movement (REM) and non-REM sleep. The cycle starts over every 80 to 100 minutes. Usually there are four to six cycles per night. You may wake up briefly between cycles. Sleep studies use sensors to record eye movements and brain activity, which are used to classify sleep phases and stages.
Non-REM sleep has three stages, defined by measurements of brain activity taken in sleep studies.
- Stage 1. This stage is the transition between wakefulness and sleep.
- Stage 2. When you reach stage 2, you are asleep.
- Stage 3. This stage is called deep sleep or slow-wave sleep, after a particular pattern that appears in measurements of brain activity. You usually spend more time in this stage early in the night.
Rapid eye movement (REM) sleep
During REM sleep, your eyes twitch and your brain is active. Brain activity measured during REM sleep is similar to your brain’s activity during waking hours. Dreaming usually happens during REM sleep. Your muscles normally become limp to prevent you from acting out your dreams. You usually have more REM sleep later in the night, but you do not have as much REM sleep in colder temperatures. This is because, during REM sleep, your body does not regulate its temperature properly.