REM sleep: the stage dreams come from

REM sleep is the stage most people mean when they say "dreaming". Its name comes from the most visible thing about it: under closed lids, the eyes move rapidly, in bursts, as if tracking something. Everything else about the stage is odd in the same direction. The EEG looks much more like waking than like the deep sleep that preceded it, breathing and heart rate become irregular, and the brain is metabolically busy — while the body lies almost completely still.

REM was discovered in 1953 by Eugene Aserinsky and Nathaniel Kleitman. Aserinsky, then a graduate student in Kleitman's laboratory at the University of Chicago, noticed regular periods of eye movement in sleeping subjects and — crucially — found that waking people during those periods produced vivid dream reports far more often than waking them at other times. That single observation turned dreaming from a subject for philosophy and psychoanalysis into something measurable on an instrument, and essentially every article in this section descends from it.

The body is switched off during REM, and that is the point. Voluntary muscles enter a state called atonia: they are actively inhibited, so the motor commands generated during a dream do not reach the limbs. You can dream about running without moving. This is the mechanism that keeps dreaming safe, and it is worth understanding because two well-known phenomena are what happens when it misfires. When atonia persists into wakefulness, you get sleep paralysis — awake, aware, and unable to move. When atonia fails during REM itself, you get REM sleep behaviour disorder, in which people physically act out dreams; that one is a medical matter and is covered separately in this section.

The eyes are the exception. Eye muscles are not paralysed along with the rest, which is why they are visible from outside — and why they later became the channel researchers used to communicate with lucid dreamers, who can signal from inside a dream with pre-agreed eye movements. The one part of the body REM leaves free turned out to be the one part science needed.

REM is also distinctive chemically. Noradrenaline levels drop to their lowest point of the entire 24-hour cycle, and acetylcholine activity is high. That combination is unusual, and it comes up repeatedly elsewhere on this site: it is part of why dreams feel emotionally intense but are so poorly remembered, and it underpins the proposal that REM offers a kind of overnight emotional processing.

The most important nuance is one that popular articles routinely get wrong: dreaming is not exclusive to REM. The early evidence appeared to make REM and dreaming synonymous, and the equation stuck in public understanding. It does not survive careful testing. Waking people from non-REM sleep also produces dream reports — David Foulkes's work was central in establishing this — and once researchers stopped asking only about vivid narratives and started asking simply what was going through the sleeper's mind, non-REM reports turned out to be common. They are typically shorter, less bizarre, less story-shaped and more thought-like, but they are dreams by any reasonable definition. Any account that treats REM as the sole home of dreaming is one generation of research out of date.

What REM is for is still genuinely open. It is heavily associated with procedural memory and with emotional processing; it is present in newborns in far greater proportion than in adults, which suggests a developmental role; and it appears in some form in other mammals and birds. But these are associations and reasonable inferences rather than a proven function, and the field has not converged on a single answer.

For a reader interested in dreams, the practical takeaways are simple. Most of the vivid, plotted, strange dreams you remember came out of a REM period, most likely the long one before waking. The reason you did not thrash around while it happened is atonia. And the reason a dictionary of symbols and a sleep laboratory are talking about the same thing at all traces back to two researchers in 1953 noticing that closed eyes were moving.

Frequently asked

Dream symbols on this topic

• ☽ Falling dream meaning — interpretation
• ☽ Flying dream meaning — interpretation

What this is based on

• Aserinsky and Kleitman's 1953 discovery of REM sleep
• David Foulkes's research on dreaming outside REM sleep
• Sleep-medicine literature on REM atonia and its disorders