What happens during sleep? (2024)

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