REM sleep explained
REM is where most dreaming happens, and it clusters in the second half of the night, which is why cutting sleep short costs you REM disproportionately.
The short answer
REM is roughly 20 to 25 per cent of adult sleep, about 90 to 120 minutes across a normal night.
It is not evenly spread. REM periods get longer as the night goes on, so most of your REM happens in the final few hours.
Where REM sits in the night
Sleep runs in cycles of roughly 90 minutes, and each cycle moves through lighter sleep, deep sleep and REM.
The proportions shift across the night in opposite directions:
Deep sleep dominates the first half. The first two cycles carry most of it, and by the early hours there is little left.
REM dominates the second half. The first REM period may last only a few minutes; by the final cycle it can run half an hour or more.
That asymmetry has a practical consequence. Cutting an hour off the end of the night removes mostly REM, while a late bedtime with a fixed wake time costs you deep sleep first. They are not the same loss.
What REM appears to do
Worth being careful here, because sleep function is an area where confident claims outrun the evidence.
Dreaming. Most vivid dreaming occurs in REM, though dreams also happen in other stages in a less narrative form.
Emotional processing. REM is associated with how emotional memories are consolidated and how their intensity is regulated, which is part of why poor sleep and low mood reinforce each other.
Memory consolidation, particularly procedural and emotional memory, where deep sleep is more associated with factual and declarative memory.
Physically, REM involves faster and irregular breathing and heart rate, rapid eye movement, and muscle atonia, a temporary paralysis of most voluntary muscles that stops you acting out dreams.
Muscle atonia, and sleep paralysis
That paralysis explains an experience a lot of people find frightening.
Sleep paralysis is the atonia of REM persisting briefly as you wake, so you are conscious but cannot move for a few seconds to a couple of minutes. It is common, not dangerous, and more likely with irregular sleep, sleep deprivation and sleeping on your back.
The opposite problem, acting out dreams physically, is a recognised sleep disorder and is worth discussing with a GP rather than ignoring, particularly in older adults.
What reduces REM
Alcohol. The clearest effect on this page. It suppresses REM in the first part of the night, and as it metabolises there is a rebound with lighter, more fragmented sleep. That is why a nightcap shortens the time to fall asleep and produces 4am waking.
Short sleep. Because REM clusters late, a six hour night costs you a disproportionate share of it rather than a proportionate one.
Irregular wake times. Weekend lie-ins shift the body clock, which desynchronises when REM is scheduled to occur.
Some medications, including certain antidepressants, suppress REM. That is a matter for the prescribing clinician rather than something to self-manage.
Fragmentation. Anything that wakes you repeatedly, including being too warm, prevents cycles completing.
Can you get more REM?
Not directly, and the indirect route is the same advice as everything else.
Sleep long enough for the later cycles to happen. This is the entire mechanism. REM is back-loaded, so the only reliable way to get more is to not truncate the night.
A consistent wake time, seven days a week, which keeps the body clock aligned so cycles run when they should.
Reduce alcohol, particularly close to bed.
Stop the fragmentation. Temperature is the most common culprit and the most fixable: 7 to 10.5 tog suits most modern UK bedrooms, and 13.5 is a convention from draughtier housing.
Ignore products claiming to increase REM specifically. No mattress, app, supplement or device targets a sleep stage, and consumer trackers infer stages from movement and heart rate rather than measuring brain activity, so the number they report is an estimate.
When broken sleep is worth investigating
If you sleep a full night and still wake unrefreshed, the issue is usually fragmentation rather than duration, and something is interrupting cycles.
Pauses in breathing during sleep, gasping awake, loud snoring or heavy daytime sleepiness need a GP, because sleep apnoea fragments sleep without waking you enough to remember it.
If falling or staying asleep has been the problem for weeks rather than nights, cognitive behavioural therapy for insomnia is the evidence-based first-line treatment in the UK and a GP can refer you.
Related reading
How sleep works
Stop the fragmentation
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Questions people ask
How much REM sleep do you need?
Roughly 20 to 25 per cent of adult sleep, about 90 to 120 minutes a night. It is not evenly spread: REM periods lengthen as the night goes on, so most of it happens in the final few hours.
Why does cutting sleep short affect REM most?
Because REM is back-loaded. The first REM period may last only a few minutes and the last can run half an hour or more, so an hour lost from the end of the night removes mostly REM.
Does alcohol affect REM sleep?
Yes, clearly. It suppresses REM early in the night, and as it metabolises there is a rebound with lighter, more fragmented sleep, which is why a nightcap produces early waking.
Can you increase REM sleep?
Not directly. Sleep long enough for the later cycles to happen, keep a consistent wake time, reduce alcohol and stop whatever is fragmenting the night. No product targets a sleep stage.