Most skeletal muscles become temporarily inactive during normal REM sleep.

REM sleep combines an active brain with strongly reduced activity in many voluntary muscles. The mismatch is normal: vivid dream experiences can occur while the arms and legs remain largely still. Muscle atonia is therefore part of a specific sleep state, not evidence that the brain has switched off.

Brainstem mechanisms contribute to the inhibition of muscles involved in posture and limb movement. By suppressing much of their activity, normal REM atonia reduces the chance that a sleeping person will physically carry out the movements occurring within a dream.

The suppression is selective rather than absolute. The eyes can move rapidly beneath closed eyelids, and breathing continues. Brief muscle twitches can also occur, so descriptions such as "temporary paralysis" should not be interpreted as complete silence in every skeletal muscle.

NINDS divides sleep into two broad categories: non-REM sleep and REM sleep. Non-REM sleep has three stages, ranging from the transition out of wakefulness to deeper sleep. A typical night cycles through these states repeatedly rather than passing through a single continuous period of uniform unconsciousness.

The first REM period usually occurs about 90 minutes after falling asleep. Later REM periods generally become longer as the night progresses. These timings describe a typical pattern, not an exact schedule that every sleeper follows. Age, sleep history, and other circumstances can change how sleep stages are distributed.

During REM sleep, brain-wave activity becomes more similar to waking activity. Breathing can become faster and more irregular, and heart rate and blood pressure can rise toward waking levels. At the same time, the muscles of the arms and legs become temporarily inactive. Different body systems are therefore changing in different directions.

Most dreaming occurs during REM sleep, although dreams can also happen during non-REM sleep. That distinction matters: REM is not simply another word for dreaming. It is a physiological state identified through patterns of brain activity, eye movements, and muscle tone, not merely through whether someone remembers a dream after waking.

Occasionally, the transition between REM sleep and wakefulness is imperfectly synchronized. In sleep paralysis, a person becomes aware while being briefly unable to move or speak. This can be frightening, but it is different from the usual unnoticed muscle atonia of REM sleep. Recurrent or distressing episodes warrant discussion with a clinician.

A different problem occurs in REM sleep behavior disorder, when normal muscle inhibition is impaired and people may act out dreams. Movements can injure the sleeper or a bed partner. Repeated shouting, punching, kicking, or falling from bed during sleep deserves medical assessment rather than being dismissed as an ordinary consequence of vivid dreaming.

NINDS describes sleep using several physiological features rather than a single scale from inactive to active. REM demonstrates why that matters: brain activity, eye movement, breathing, and limb-muscle tone do not all shift together. A quiet-looking sleeper can have substantial internal activity. The absence of large movements therefore cannot tell an observer everything about the person's current sleep state.