Rapid eye movement (REM) sleep is sometimes thought of as a paradox because the brain is very active, but the body remains completely still.

The brain during REM sleep

During earlier stages of non-REM sleep, your brain’s activity becomes increasingly slower. Upon entering REM sleep, brain activity speeds up again, becoming similar to the activity seen during your waking hours. This can lead to complex, vivid dreams.

The body during REM sleep

As your body falls into REM sleep, many of your muscles become temporarily paralyzed. This is important for keeping you in place as you sleep — for example, it prevents your legs from kicking out as you dream about running.

Certain muscles continue to move, however. The muscles that help you breathe remain active, and the muscles in your eyes cause your eyes to quickly dart from side to side.

Medically reviewed by

Dr. Colleen Ehrnstrom's profile picture
Dr. Colleen Ehrnstrom, PhD

Dr. Colleen Ehrnstrom is a licensed clinical psychologist with a specialty practice in Acceptance and Commitment Therapy (ACT). Areas of expertise include insomnia and other sleep disorders, anxiety, and depression.

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