What is REM sleep?

Maybe it was the alarm breaking into a dream, or a dream so real you reached for your phone to check whether that message was ever sent. Maybe it's the bar on your sleep tracker's chart that grew one night and shrank the next, or the flicker under a partner's closed eyelids near morning. Each of these is REM. In every one of them, a brain is hard at work inside a body that's lying perfectly still.

If you watched a cat go into REM sleepREM sleep Rapid eye movement sleep, often called dreaming sleep: the stage when most vivid dreaming happens. The eyes move quickly beneath the lids while most of the body’s muscles stay relaxed and still., you could see it happen: its head and legs go completely slack, its eyes dart under closed lids, its breathing turns uneven and its paws and whiskers twitch¹1 Blumberg et al., 2020. Sleep researchers recognize the same state in people by three things measured together: muscles gone slack, rapid eye movements and a cortex as active as it is when awake¹²2 Vetrivelan & Bandaru, 2023. That pairing, a brain lit up inside a body switched off, is found nowhere else in the day or night. It's why one of REM’s older names is paradoxical sleep¹.

It may help to picture two switches. One is for the brain, and it sets how busy the brain is. The other is for the muscles that move the body, and it sets whether they can move at all. Most of the time the two go up and down together. In REM they part: the brain’s switch rises nearly all the way, while the brainstem keeps the body’s switch down. So REM doesn’t belong on the scale of light and deep sleep. It's a state of its own, as distinct from the rest of sleep as it is from waking¹².

The state was first reported in 1953, when recordings of sleeping adults showed bursts of brain activity and rapid eye movements in the middle of sleep¹. Until then, scientists had pictured sleep as something passive, simply the absence of waking¹, and here was the brain in the middle of the night, busy. Early scans of sleeping people have begun to show what it's busy with. In REM the parts that handle feeling and memory are lit, along with the parts at the back of the brain that make pictures, while the front part, which plans and keeps a check on yourself, is among the least active³3 Maquet, 2000.

REM arrives four or five times a night, roughly every hour and a half to two hours⁴4 Feriante & Singh, 2024, weaving among the other stages that What happens while we sleep? describes. While it lasts, the rest of the body follows its own course. Heart rate and breathing speed up and turn irregular, and for a while the body stops regulating its temperature². Against the stillness come brief twitches, especially in the face, and bursts of eye movement², so someone lying beside you near morning might see your eyes flicker under the lids while the rest of you lies still.

The body’s switch is thrown deep in the brainstem, and it works in steps. When REM begins, a small cluster of nerve cells there switches on. It rouses other cells lower down, toward the spinal cord, and these apply a chemical brake to the nerves that drive the muscles, so the muscles fall silent²2 Vetrivelan & Bandaru, 2023. A few are spared: the diaphragm and breathing muscles, the eye muscles and the tiny muscles of the inner ear². That's why you go on breathing and your eyes go on moving while the rest of you stays put.

Work in rats and mice shows how much depends on that small circuit. When researchers remove the cells that switch on, or the ones lower down that apply the brake, the animals still enter REM but without the stillness, and they move as if acting out their dreams². Horses show the same switch from the outside. A horse can doze on its feet but has to lie down for REM, and one that won’t lie down can slip into REM standing, lose its muscle tone all at once and fall¹1 Blumberg et al., 2020.

The usual explanation for the stillness is that it keeps us from acting out our dreams while the brain’s movement areas are busy⁵5 Denis, 2018. Some researchers doubt it, because dreams come outside REM too¹. Up to 70% of wakings from non-REM sleep bring a report of a dream⁶6 Siclari et al., 2017, so REM and dreaming overlap without being one and the same. Why do we dream? picks up from there.

In the last hour before the alarm, the dreams can feel fuller than anything earlier in the night, crowded and bright and hard to leave, and that last hour is where REM gathers. Each REM period of the night runs longer than the one before⁴4 Feriante & Singh, 2024, so the longest come in the hours before you usually wake, and their timing follows your body clock more than how long you’ve been asleep⁸8 Dijk, 2009.

Researchers found this by setting sleep loose from the clock. Volunteers lived in a laboratory on twenty- or twenty-eight-hour days, so their sleep fell at every hour of the body’s day. Their deep sleep mostly followed how long they'd been awake. Their REM kept to the body clock. It peaked one to two hours after the body’s temperature reached its daily low, close to the usual time of waking⁸.

So on an ordinary night the early hours have most of the deep sleep, and the later ones are dominated by REM⁹¹⁰9 Siegel, 200110 Rasch & Born, 2013. An alarm set two hours early trims the very hours richest in it, since cutting the last hours of a night takes mainly REM⁹¹⁰. A night that starts two hours late and ends when you wake on your own probably keeps more of its REM. The clock’s REM-rich hours still arrive at their usual time, though no study has yet set the two nights side by side.

Those last dreams are also the ones best placed to stay with you. Dreams seem to be laid down in memory only if you wake, at least briefly, soon after them. Those not thought over on waking fade fast⁹¹¹11 Eichenlaub et al., 2014. One line of research finds that people who recall many dreams wake more during the night¹¹. So waking straight out of one of the long REM periods of early morning, on a slow weekend morning or with the alarm cutting through the middle of a story, likely gives a dream its best chance of coming with you into the day.

Perhaps it happened only a little while ago. You came up out of sleep in the dark, aware of the room and fully yourself, and your body wouldn't answer. You couldn't so much as lift a finger. It's a hard thing to put into words for anyone the next day. About one in thirteen people have had at least one episode¹³13 Sharpless & Barber, 2011, and it has a name, sleep paralysissleep paralysis A brief spell, on falling asleep or waking, of being aware but unable to move the body, while the eyes move and breathing goes on as usual., and a plain mechanism. The stillness of REM carries on for a short while after the mind has woken. You can see and breathe and know where you are, but you can’t yet move⁵5 Denis, 2018. The brain’s switch has come up and the body’s hasn’t followed yet, and within a few seconds or a few minutes, movement comes back⁷7 Bhalerao et al., 2024.

What can make it frightening is what often comes with it. Most episodes bring hallucinations, often a sense of someone in the room or a pressure on the chest⁵. One proposal is that the brain’s fear circuits, which are active in REM, stay on for a moment after you wake, with nothing in the room to explain them⁵. It's only a proposal so far, since nobody yet knows which brain regions are involved. Not every episode is frightening, either: in one survey as many as one in five people who’d had it had also had pleasant episodes⁵.

Sleep paralysis is also a common feature of narcolepsy, a rare neurological condition⁵. That's a different matter, and What is narcolepsy, and could I have it? covers it. For most people, sleep paralysis is an occasional thing. Only about one in two hundred healthy people have it several times a week, and it's linked with short or broken sleep, jet lag, shift work, stress and anxiety⁵¹³. Each time, the body’s switch comes back up, and you can move again.

More than sixty-five years after REM was found, researchers still don’t agree on what it's for. They've proposed many theories and ruled out few¹1 Blumberg et al., 2020. Memory is the idea you hear most, and both sides of that argument have good evidence to point to.

The case for memory is strongest in animals. In many studies in rats, mice and cats, REM rises after learning, and keeping the animals out of REM impairs their learning of complex tasks¹⁰10 Rasch & Born, 2013. The memory-side reviewers themselves note that many of those studies kept the animals from REM in stressful ways¹⁰. In people the picture is patchier. The clearer findings are for skills, such as a visual task that improved overnight unless REM was blocked¹⁰, and reviewers who favor a memory role call the evidence in people surprisingly inconsistent¹⁰.

The strongest evidence on the other side comes from people too, and one of them is hard to forget. One man was left with little or no REM after a shrapnel wound to his brainstem. He went on to finish law school, and then to practice law⁹9 Siegel, 2001. He's one person, so his case can't decide the question alone. Still, law school is a hard place to get through on a memory that isn't working. Then there are the drugs. Some older antidepressants can abolish REM for months or years, and the millions who've taken them show no sign of memory loss⁹. Nobody tested those millions carefully, so this is a matter of noticing what didn’t happen.

Today’s antidepressants keep the question alive. Many common ones, SSRIs and SNRIs among them, delay and reduce REM¹⁴14 Wichniak et al., 2017. On one point the two sides meet: even researchers who favor a memory role accept that losing REM on antidepressants doesn’t bring obvious memory problems¹⁰. They part over why. One side suggests the drugs’ own chemistry may stand in for whatever REM does¹⁰, and skeptics reply that the drugs push brain chemistry the opposite way to REM⁹.

That shared ground says something plain. Whatever REM does for memory in people, it isn’t something anyone visibly misses when it’s gone. It may still do something subtler, and the animal work keeps that possibility alive, so what REM is for stays an open question. These are findings about what REM does, and they leave anyone’s treatment where it is. If you take one of these medicines and have questions about it, your prescriber is the person to ask.

Perhaps you've looked at a tracker after a late night or a few drinks, seen the REM bar shorter than usual, and wondered whether something was lost for good. (How well a tracker can tell REM from the rest of sleep is a question of its own, taken up in What is deep sleep, and am I getting enough?) The brain notices a loss of REM and makes some of it up. People woken each time REM begins try to enter it more and more often, and on the nights that follow, REM comes back in greater amounts than usual⁴⁹4 Feriante & Singh, 20249 Siegel, 2001. That return is called REM reboundREM rebound A return of extra dreaming sleep after it has been held back, for example by alcohol or a short night., and the same rebound follows stopping medicines that suppress REM, or alcohol withdrawal⁴.

An early idea held that people kept out of REM would begin to hallucinate or fall ill. In 1975 a researcher named Vogel gathered the experiments together, and the review found REM deprivation was not harmful to healthy people, nor to people with depression or schizophrenia⁹¹⁸18 Vogel, 1975. Those were short laboratory studies, and a night with less REM than usual is the kind of loss they looked at.

Does every animal make up lost REM? One study of fur seals suggests not. Researchers kept four fur seals in seawater for ten to fourteen days, and their REM fell from about eighty minutes a day to three. Back on land, the seals made up almost none of it¹⁹19 Lyamin et al., 2018. The researchers wonder whether REM partly serves to rewarm the brain after deep non-REM sleep, which might also be why it grows toward the end of the night¹⁹. It's an idea from four seals, and it waits to be tested in people.

What REM is for may take years more to work out, but what it is fits in one picture. In the last hours of the night, the body clockbody clock The timekeeper in your brain that runs on a cycle of about a day, setting when you feel sleepy and when you feel alert. Daylight, especially in the morning, keeps it in step with the clock on the wall. turns toward morning. The parts of the brain that handle feeling and pictures light up, and the eyes move behind closed lids. Almost every muscle that moves you goes slack, its switch set apart from the brain’s for a while, the same way it is in the dog paddling its paws on the kitchen floor. Most mornings the two come back into step as you wake, and you bring a dream out with you, or nothing at all. Once in a while one lags a few seconds behind the other, and then it catches up.