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Open interactive - The state of sleep. What happens during sleep?

reviewed 12/6/2019


What happens in your brain and body as you sleep.

You may be lying at rest while you sleep, but there’s an invisible flurry of activity going on inside your brain and body. Muscles are being repaired, hormones are being released and memories are being solidified. All of this happens during a series of 90-minute cycles of varying stages of sleep.

Getting a good night’s sleep with a balance of each stage of sleep will help you to wake up feeling refreshed and ready for the coming day. 

So what happens while you sleep?

There are two basic types, or states, of sleep: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep, which includes three stages.

The first NREM stage

This first NREM stage is a time of drowsiness and transition between wakefulness and sleep. During this stage of light sleep, you can be easily awakened by a slight sound or touch.

You might also experience a falling sensation followed by sudden muscle jerks.

This stage lasts for about 10 minutes.

You spend more time in the second NREM stage than in any other sleep phase.

The second NREM stage

This stage is also a time of light sleep. You tune out from the world around you. Your body temperature drops, your heart rate slows, your eyes stop moving and your muscles relax further. Your brain waves become slower but have brief bursts of electrical activity called “sleep spindles.”

The third NREM stage

Now comes the third NREM phase of sleep, sometimes called “deep sleep” or “slow-wave sleep.” You need this period of deep sleep to feel refreshed in the morning.

Brain waves slow down even more during this phase and it may be hard to wake you up. And if you are woken up during this stage, you might feel groggy or disoriented for several minutes.

During this third stage:

  • Heartbeat and breathing slow to their lowest levels during sleep.
  • Blood pressure drops.
  • Blood supply to the muscles increases.
  • Tissue growth and repair occurs.
  • Essential hormones are released.
  • Energy is restored.

The longer you've been awake, the more time you spend in this stage of sleep.

Your brain paralyzes many of your muscles during REM sleep, which stops you from acting out your dreams. This state of paralysis is called "atonia."

REM sleep

REM sleep is the last stage of every sleep cycle. Although your eyes are closed, they flicker back and forth rapidly. Your brain waves speed up. Most of your dreams occur during this phase. You spend a total of about 25 percent of sleep time in REM sleep.

The cycle repeats all night long

One full sleep cycle with each type of sleep lasts about 90 minutes. After the REM sleep stage ends, you start the cycle all over again, beginning with the first stage of NREM sleep. Most adults will go through four to six sleep cycles in a full night of sleep.

How many hours of sleep do you need each night?


Sources: American Academy of Sleep Medicine; National Institute of Neurological Disorders and Stroke; National Institutes of Health; National Sleep Foundation

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