In dreaming sleep, your brain is as busy as when awake, but you're paralysed.
Several times a night your eyes start darting beneath closed lids, your brain burns energy as fast as it does when you are awake, and almost every muscle in your body goes limp. This is REM sleep, described in 1953, and scientists still argue about what it is for.
In 1953 Nathaniel Kleitman and his student Eugene Aserinsky described rapid eye movement sleep and linked it to dreaming; researchers including William Dement and Michel Jouvet later filled in the picture. Sleep turned out to be a cycle rather than a single state. In adults, deeper non-REM sleep and REM alternate roughly every 90 minutes, with REM periods lengthening from about 15 minutes early in the night to about 25 minutes towards morning. In total, REM takes up around a fifth to a quarter of adult sleep.
It is also called paradoxical sleep, because the brain looks awake. Its electrical waves are fast and small, like waking ones, and its energy use equals or exceeds that of wakefulness. Meanwhile the body is almost completely paralysed, a state called REM atonia. When that paralysis fails, people can physically act out their dreams, a condition known as REM sleep behaviour disorder. The body also partly stops regulating itself: breathing and heart rate turn irregular, and temperature control largely switches off.
Woken during REM, about 80 per cent of people can report a dream, often vivid and story-like. But dreams also happen outside REM, and some researchers dispute how tightly the two are tied. Even the eye movements are puzzling. They are generally slower than waking eye movements, and people blind from birth, who typically do not dream in pictures, still make them.
Newborns spend more than 80 per cent of their sleep in REM, which fits the idea that it helps wire the developing brain. Other theories link it to consolidating memories, especially of skills, and to creative thinking: people woken from REM do better on anagrams. Yet the case is far from closed. One person left with almost no REM sleep after a brainstem injury showed no memory problems. REM or something like it occurs in all land mammals and in birds, which suggests it matters, even if we cannot yet say exactly why.
Source: Wikipedia — Rapid eye movement sleep · Text summarised from Wikipedia (CC BY-SA 4.0)
