REM sleepRapid eye movement sleep · Paradoxical sleep
The dreaming stage of sleep, important for memory consolidation and emotional regulation.
REM sleep is the stage named for the rapid eye movements that accompany it. The brain is highly active, close to waking levels on an EEG, while the body's voluntary muscles are effectively paralysed, which is thought to stop you acting out what you are dreaming.
It occupies roughly a fifth to a quarter of an adult night and is distributed unevenly. Early cycles contain mostly deep slow wave sleep, later cycles progressively more REM, so the last few hours before waking are REM heavy. Cutting a night short at the end removes disproportionately more of it.
Its functions are still being mapped, but the evidence points towards emotional processing, procedural and skill memory consolidation, and creative recombination of information. Sleeping on a problem is not folklore, though the mechanism is less tidy than the phrase suggests.
Several common things suppress it. Alcohol suppresses REM in the first half of the night and produces rebound in the second, which is part of why a drinking night ends in fragmented early morning sleep. Some antidepressants suppress it substantially, and cannabis does too, with rebound on stopping.
Vivid recall of dreams usually just means you woke during or shortly after a REM period, which is why it is common with fragmented sleep or an alarm at the wrong moment rather than a sign of unusual sleep quality.
Consumer trackers report REM and report it unreliably. Validation studies find total sleep time reasonably accurate and stage estimates poor, which makes chasing a REM percentage an exercise in optimising a number the device cannot really measure.
In practice, protect total sleep time and a consistent schedule, and the architecture arranges itself.