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What happens during sleep?

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When a person is sleeping, the brain cycles through five distinct phases: stage 1, 2, 3, 4 and rapid eye movement (REM) sleep. Each phase helps to ensure that the mind and body are rested. Certain phases are needed to help you feel rested and energetic the next day, while other phases help you learn information and form memories.1, 2

Sleep phases progress in a cycle from stage 1 to REM sleep, and then the cycle starts over again with stage 1. Some of the characteristics of each phase are shown below:

  • Non-REM sleep (75% of sleep): As you begin to fall asleep, you enter non-REM, which consists of stages 1 through 4, as follows: 1, 2
    • Stage 1
      • You are between being awake and falling asleep.
      • You may start to lightly sleep.
    • Stage 2
      • You fall into a deeper sleep.
      • You become disengaged from your surroundings.
      • Your body temperature becomes slightly lower.
    • Stages 3 and 4
      • Your deepest and most restorative sleep occurs.
      • Your blood pressure drops.
      • Your breathing rate slows.
      • Your muscles relax.
      • Your body increases the supply of blood to your muscles.
      • Your body performs tissue growth and repair.
      • Your energy is restored.
      • Your body releases hormones.
  • REM sleep (25% of sleep): Approximately 70 to 90 minutes after you fall asleep, and then at successive intervals of about 90 to 110 minutes, you enter REM sleep. REM sleep becomes longer later into the night. REM is characterized as follows:1, 2
    • Your brain and body are energized.
    • Your daytime performance is supported.
    • Your brain is active, and dreaming occurs.
    • Your eyes dart back and forth.
    • Your body becomes immobile and relaxed.
    • Your body temperature is not as tightly regulated.

The duration of sleep phases change during a given night's sleep. For example, near the beginning of a night of sleep, the body cycles through relatively short periods of REM sleep and long periods of deep sleep. As time proceeds throughout the night, 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.2


  1. National Heart, Lung, and Blood Institute. (2009). At-a-glance: Healthy sleep. Retrieved May 30, 2012, from http://www.nhlbi.nih.gov/health/public/sleep/healthy_sleep_atglance.pdf (PDF - 1.81 MB) [top]
  2. National Institute of Neurological Disorders and Stroke. (2007). Sleep: A dynamic activity. In Brain basics: Understanding sleep. Retrieved May 29, 2012, from http://www.ninds.nih.gov/disorders/brain_basics/understanding_sleep.htm#dynamic_activity [top]

Last Updated Date: 07/31/2013
Last Reviewed Date: 07/09/2013
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