What happens during sleep?

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.

Each phase of sleep helps the mind and body stay rested. Certain stages help you feel rested and energetic the next day, while both phases help you learn information and form memories.1,2

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.

As you begin to fall asleep, you enter non-REM sleep, which consists of stages 1 through 3, as follows:1,2

  • Stage 1
    • You are in between being awake and being asleep.
    • Your heartbeat and breathing slow and your muscles relax.
  • Stage 2
    • You are in a light sleep.
    • Your brain waves slow down.
    • Your body temperature lowers.
  • Stage 3
    • Your deepest and most restorative sleep happens.
    • Your heartbeat and breathing slow to their lowest levels.
    • 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.

You first enter REM sleep about 90 minutes after you fall asleep. REM sleep becomes longer later into the night.2 REM is characterized as follows:1,2

  • Your brain and body are energized.
  • Your breathing becomes fast and irregular.
  • Your brain is active and dreaming occurs.
  • Your eyes dart back and forth.
  • Your body becomes immobile and relaxed, preventing you from acting out your dreams.
  • Your body temperature is not as tightly regulated.4

REM sleep begins in response to signals sent to and from different regions of the brain. Signals are sent to the brain’s cerebral cortex, which is responsible for learning, thinking, and organizing information. Signals are also sent to the spinal cord to shut off movement, creating a temporary inability to move the muscles (“paralysis”) in the arms and legs. If this temporary paralysis is disrupted, people might move while they are dreaming (“sleepwalking”). A person who sleepwalks is at risk for injury.5

REM sleep is thought to be involved in storing memories, learning, and balancing mood.6 REM sleep stimulates regions of the brain that are used for learning. Studies have shown that when people are deprived of REM sleep, they are not able to remember what they were taught before going to sleep.1 Lack of REM sleep has also been linked to certain health conditions, such as migraines.6 However, insufficient sleep, regardless of sleep stage, can interfere with learning, memory, and performance. If you have any concerns about your sleep quality and habits, speak with your healthcare provider.

Scientists aren’t sure why we dream. While some of the signals sent to the cortex during sleep are important for learning and memory, some signals seem to be random. Dreams are generally most vivid during REM sleep, but dreaming can also occur during non-REM sleep.1

Through research, we are learning more about dreaming. One study, for example, found that a pattern of brain activity from a part of the cortex near the back of the brain is a good predictor of whether an individual is dreaming, whether the individual was in the REM or non-REM sleep.7


  1. National Institute of Neurological Disorders and Stroke. (n.d.). Brain basics: Understanding sleep. Retrieved June 7, 2017, from https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Understanding-Sleep
  2. National Sleep Foundation. (n.d.). What happens when you sleep? Retrieved May 1, 2018, from https://sleepfoundation.org/how-sleep-works/what-happens-when-you-sleep 
  3. Stevens, M.S. (2015). Normal sleep, sleep physiology, and sleep deprivation. Medscape. Retrieved June 27, 2017, from http://emedicine.medscape.com/article/1188226-overview?pa=lfFeHjbS6JaS25ErG2lkd%2BxXm76%2Bmr3oCutNnyc%2FQBHaPwezUnAfojLlwoYnh8sJNFsYxDuz%2Fz2hge3aAwEFsw%3D%3D 
  4. National Sleep Foundation. (n.d.). Does your body temperature change while you sleep? Retrieved June 26, 2017, from https://sleep.org/articles/does-your-body-temperature-change-while-you-sleep/ 
  5. National Sleep Foundation. (n.d.). Sleepwalking. Retrieved June 12, 2017, from https://sleepfoundation.org/sleep-disorders-problems/abnormal-sleep-behaviors/sleepwalking 
  6. National Sleep Foundation. (2010). REM sleep deprivation and migraines. Retrieved June 12, 2017, from http://www.sleepfoundation.org/alert/rem-sleep-deprivation-and-migraine 
  7. Siclari, F., Baird, B., Perogamvros, L., Bernardi, G., LaRocque, J.J., Riedner, B. … Tononi, G. (2017). The neural correlates of dreaming. Nature Neuroscience, 20(6), 872–878. Retrieved April 22, 2019, from https://www.ncbi.nlm.nih.gov/pubmed/28394322
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