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

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.

How do our patterns of sleep change as we age?

The patterns and types of sleep change as people mature. For example, newborns spend more time in REM sleep. The amount of slow-wave sleep peaks in early childhood and then drops sharply in the teenage years. Slow-wave sleep continues to decrease through adulthood, and older people may not have any slow-wave sleep at all.

NHLBI research on sleep

Sleep is one of the four primary areas of NHLBI research. Learn more about the ways the NHLBI is advancing understanding of sleep and sleep disorders. Research on this topic is part of the NHLBI’s broader commitment to advancing scientific discovery for sleep science.

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