REM sleep
REM sleep is a recurring sleep phase in which the eyes move rapidly beneath closed lids and most vivid dreams occur. During this phase, the brain is almost as active as when awake, while the body's muscles are largely paralyzed.
Sleep is not a uniform state, but proceeds in rounds. One of these rounds is called REM sleep. The abbreviation stands for “Rapid Eye Movement”: the eyes twitch back and forth beneath the closed lids. Most vivid dreams occur during this phase. The brain works nearly as hard as when awake, yet the muscles of the arms and legs are temporarily paralyzed. Because of this contradiction, REM sleep is also called paradoxical sleep.
What the brain does at night with the day
REM sleep is not a break, but working time for the brain. Research suggests that here experiences are sorted and linked with older knowledge. Particularly emotional memories seem to be defused during this phase. People who sleep poorly react measurably more irritably to small things the next day.
REM sleep also plays a role in learning. After a night spent cramming, less sticks than after a normal night with the same amount of evening study time. Sleep is therefore not a lost part of the day, but belongs to the learning process itself. This is one reason why experts advise against cramming all night before exams.
A common misconception is that dreams only occur during REM sleep. Dreaming also happens in the other phases, though usually more faintly and less story-like. REM sleep provides the dreams one remembers in the morning.
The course of a night
A night consists of about four to six sleep cycles. Each cycle lasts roughly 90 minutes. It begins with light sleep, moves into deep sleep, and ends with a REM phase. Then the next cycle begins.
The REM phases are not all the same length. At the start of the night they last only a few minutes, but toward morning sometimes half an hour. In total, an adult spends about 20 percent of the night in REM sleep. In newborns it is about half, which suggests a role in brain development.
The muscle blockade during REM sleep is a protective function. It prevents a person from actually carrying out dream movements. In some people this blockade doesn’t work properly, and they hit or kick around in their sleep. Doctors then speak of REM sleep behavior disorder. Anyone who is briefly unable to move upon waking consciously experiences this paralysis.
REM measurement in watches and sleep apps
Smartwatches and fitness bands often show a chart in the morning with deep sleep, light sleep, and REM shares. These devices do not measure REM sleep directly. They record movement, pulse, and sometimes skin temperature. From this, an algorithm — that is, a fixed computational procedure in the software — estimates the sleep phase.
In a sleep lab, things work differently. There, electrodes are attached to the head and record the brain’s electrical activity, as well as eye movements and muscle tension. This measurement is called polysomnography and is considered the standard. Comparative studies show that watches estimate total sleep time fairly well, but the individual phases much less accurately.
In tech news, REM sleep therefore frequently appears in connection with AI models. Manufacturers train models to predict the results of a sleep lab from sensor data. The values are mainly useful as a rough trend over weeks. A single night with a low displayed REM share is no cause for concern.