REM sleepRapid eye movement sleep · Paradoxical sleep

REM sleep is characterised by rapid eye movements, brain activity resembling wakefulness, and near-complete muscle paralysis that stops you acting out dreams. It typically accounts for around a fifth to a quarter of a night's sleep, concentrated in the later cycles.

It matters because it appears central to consolidating procedural and emotional memory and to processing emotional experience. REM deprivation impairs learning and measurably worsens emotional regulation, and its role in motor-skill consolidation makes it relevant to anyone learning movement.

In practice REM is suppressed by alcohol, by cannabis and by several antidepressants — which is one clear reason a nightcap produces sleep that is longer but less restorative. Because REM is loaded towards the end of the night, cutting sleep short costs it disproportionately.