REM sleep
Meaning of REM sleep: The stage where the brain is busiest and the body stillest, held slack while the eyes dart behind closed lids.
Definition of REM sleep
REM sleep is the stage of sleep in which the eyes move rapidly beneath closed lids, brain activity resembles wakefulness, and vivid dreaming is most common. The muscles of the limbs and trunk are held almost completely slack throughout it, so the body stays still while the mind is active. It returns several times across a night.
What happens in the body during it?
The brain wave pattern flattens and speeds up until it resembles that of someone awake, which is why the state was once called paradoxical sleep. Breathing and heart rate become irregular. Blood flow to the brain rises. Temperature regulation is largely suspended, so shivering and sweating stop and body temperature drifts toward that of the room. Meanwhile a signal from the brainstem paralyzes the voluntary muscles, sparing the eye muscles and the diaphragm, so breathing continues while movement does not.
Where does it sit among the stages?
| Stage | What it is like |
|---|---|
| N1 | Drifting off, the lightest stage, easily interrupted |
| N2 | The bulk of the night, with characteristic bursts of brain activity |
| N3 | Deep slow-wave sleep, the hardest to rouse someone from |
| REM | Active brain, slack body, most vivid dreaming |
Older texts describe five stages because deep sleep was once split in two. Current scoring recognizes three non-REM stages plus REM, so a modern account of the sleep stage sequence has four rather than five.
Why does the paralysis matter?
It prevents dreams from being acted out. When the mechanism fails, the result is REM sleep behavior disorder: shouting, punching, kicking and falling out of bed while dreaming, sometimes with injury to a bed partner. It is one of the more strongly predictive findings in medicine, because a large proportion of people who have it later develop Parkinson disease or a related condition, often many years afterward. The reverse situation, in which the paralysis lingers for a few seconds after waking, is sleep paralysis, an unsettling but harmless experience. Both sit in the classification of sleep disorder categories, and rapid eye movement sleep is simply the full form of the name.
What suppresses it?
Alcohol shortens it early in the night and is followed by a rebound of intense dreaming in the small hours. Many antidepressants reduce it substantially. After a period of sleep loss the body makes up this stage preferentially once sleep resumes, which is one reason recovery sleep feels unusually full of dreams.
Also known as
Used in a sentence
The sleep study showed loss of the normal muscle atonia during REM sleep.
Good to know
REM is not the lightest stage of sleep; the first non-REM stage is, and REM is better described as a separate state in which an active brain is paired with a paralyzed body.