REM sleep: what it is, when it happens and why your tracker’s figure is only an estimate

What is REM sleep?

REM stands for rapid eye movement. The Royal College of Psychiatrists describes it as one of the two main types of sleep, alongside non-REM sleep, and says it comes and goes through the night.¹ It makes up about one fifth of your sleep time.¹

During REM sleep the brain is very active, the muscles are very relaxed and the eyes move from side to side.¹ The Sleep Charity adds that brain activity becomes close to that of being awake, and that this is when most dreaming happens.²

The word “most” matters. The sources read do not say that all dreaming happens in REM sleep, so this guide does not say so either.²⁴

This page is about REM sleep on its own. For the full set of stages and how a night is built, see sleep stages explained. For the stage that gets compared with REM most often, see how much deep sleep you need.

Where does REM fit in a night?

A sleep cycle lasts about 90 minutes in adults,¹ and The Sleep Charity says a good night usually includes four to five full cycles.² REM comes at the end of each cycle, after the non-REM stages.¹²

The cycles are not identical. The Royal College of Psychiatrists says the first two cycles hold a lot of deep sleep, while the last ones hold very little deep sleep and a lot of light sleep.¹ It also says that most of our REM, or dreaming, sleep comes in the morning, which is why nightmares are more common then.¹

The Sleep Charity puts the other half of the picture in the same way: the deepest sleep comes earlier in the night and lighter sleep towards the morning.² It also says dreams generally become more vivid as REM progresses.⁴

The practical point follows from the table. Because REM is weighted towards the end of the night, a night cut short at the morning end is likely to lose more REM than a night cut short at the start. That is an inference from the pattern the College describes, not a measured figure, and none of the sources read puts a number on it.

How much REM sleep do you need?

The honest answer from the British sources read is that there is no fixed number. The Royal College of Psychiatrists does not set a REM target. It says to judge your sleep by how you feel in the day, and that if you feel well, alert and rested most of the time you are getting enough sleep for you, regardless of how many hours you sleep.¹

The only share the College gives is “about one fifth” of your sleep time.¹ You will see tighter percentages and minute ranges elsewhere online. None of the sources read for this guide gives them, so they are not repeated here.

A worked example: what one fifth looks like

Take someone who sleeps 7 hours, which is 420 minutes. One fifth of that is 420 × 0.2 = 84 minutes. For 8 hours of sleep, 480 × 0.2 = 96 minutes. These are averages worked out from one rounded fraction, not targets, and a normal night can sit well away from them.

Two cautions apply. First, these figures count minutes asleep, not minutes in bed, so time spent awake comes off the total first. Second, because the fraction is rounded to “about”, the minutes should be read as roughly 80 to 100 and no closer.

If you want to know whether you are sleeping enough overall, the question to start with is total sleep, not REM. See how much sleep you need. The College also says to speak to your GP if you sleep far more or far less than the recommended hours for your age.¹

What does REM sleep do? What is known and what is debated

The Sleep Charity links REM sleep to learning, memory and emotional health.² That is a general statement of importance. It does not say how, and it is not a promise that more REM improves any of those things.

The research is more careful. A 2009 review by Matthew Walker and Els van der Helm proposed a hypothesis that REM sleep helps to process emotional memories, and linked sleep to how strongly the emotional brain reacts the next day.¹² The authors present it as a proposal, and this guide reads it that way. Only the abstract was read.

A 2025 paper from Cardiff University asked whether the eye movements of REM play a part in overnight emotional processing. Its opening point is that the function of those eye movements is still poorly understood.¹¹ Again, only the abstract was read.

The Royal College of Psychiatrists offers one theory about dreams: that they may let us practise dealing with negative events and emotions. It presents this as a theory, not a certainty.¹

• Established in the sources read: REM is a distinct stage with an active brain, relaxed muscles and moving eyes, and it makes up about a fifth of sleep.¹
• Linked to, in general terms: learning, memory and emotional health.²
• Still debated: the exact job of REM, including why the eyes move and whether REM processes emotional memories.¹¹¹²

So any page that says REM “consolidates memory” or “resets your mood” as settled fact is going further than the evidence read here.

Why your tracker’s REM figure is only an estimate

A laboratory sleep study records brain activity, and that recording (polysomnography, or PSG) is the reference that wearables are judged against in the studies read. A watch or ring does not record brain activity. It estimates the stage from indirect signals. The studies below tested specific devices, and none of them tested Elio.

The results are mixed. In a 2024 study of three commercial wearables in 35 healthy adults, sensitivity for telling the stages apart ranged from 50 to 86%. The authors found that one ring was not different from PSG for REM on a single night, and they declared links to that manufacturer.¹³

A 2025 clinical study from Charité tested three smart rings against PSG and found much lower sensitivity for REM on one of them: 0.14 for REM classification on the Circul ring. It also found that overall accuracy across all stages was 53.18% for Oura and 50.48% for SleepOn, against 35.06% for Circul, and said that group averages hide large individual errors.¹⁴

Earlier work points the same way. A 2021 study of seven consumer devices against PSG in 34 young adults described the stage assessments as inconsistent,¹⁶ and a 2019 review of Fitbit models described their estimates of sleep parameters and time in each stage as gross, of limited specificity and not a substitute for PSG.¹⁷

What this means in practice: treat the REM line on a chart as a rough guide to the shape of your night, and your own trend over weeks as more useful than any single night. The exact number of REM minutes is the part trackers are least able to give you. For devices compared side by side, see best sleep trackers, and for how an overall score is built, see sleep score explained.

What can reduce REM sleep?

Alcohol

A review of alcohol and sleep by researchers at the London Sleep Centre found that, at all doses studied, alcohol significantly delayed the first period of REM sleep. At moderate and high doses, the total share of the night spent in REM was reduced in the majority of studies, while at low doses there was no clear trend.⁹

The same review found that alcohol shortened the time to fall asleep and made the first half of the night more solid, but increased disruption in the second half.⁹ The abstract was the only part read.

The Sleep Charity says alcohol reduces dream sleep, which it calls one of the most refreshing and important stages, and causes many brief wakings that you do not remember. It adds that alcohol often makes sleep worse overall even when it helps you drop off.³ It also says that when someone stops alcohol or other substances that suppress REM, unusually vivid dreams can follow.⁴

How long before bed? The two NHS pages differ

The NHS gives two different timings. Its insomnia page advises not smoking or drinking alcohol, tea or coffee at least 6 hours before going to bed.⁶ Its Every Mind Matters page on falling asleep faster advises avoiding alcohol and nicotine 1 to 2 hours before bed.⁷ The two pages answer slightly different questions, so this guide reports both and does not average them.

Irregular hours and short nights

The NHS advises getting up at the same time every day, and not lying in after a bad night.⁶ Given that the Royal College of Psychiatrists places most REM in the morning,¹ a shortened night is the situation in which morning REM is most exposed, though no source read measures how much is lost.

Age

A meta-analysis of 65 studies covering 3,577 people aged 5 to 102 found that, in adults, the percentage of REM sleep and REM latency (the time before the first REM period) decreased significantly with age. The authors add that the size of the effects depended on how carefully participants were screened for illness, alcohol use and sleep apnoea, and in some cases reduced or hid the link with age.¹⁰ The abstract read gives the direction but not percentages by age, so none are given here.

How to protect your REM sleep

No source read offers a way to raise REM sleep on demand, and this guide does not claim one. What the sources do support is protecting the conditions in which a normal night, REM included, can happen.

• Protect the morning end of the night. Since most REM comes late,¹ getting up at the same time each day, as the NHS advises,⁶ keeps that part of the night intact. This is an inference, not a measured result.
• Be careful with alcohol close to bedtime. The evidence read links it to a delayed first REM period and more disrupted second-half sleep.³⁹ The NHS gives 6 hours on one page and 1 to 2 hours on another.⁶⁷
• Look at total sleep first. The Royal College of Psychiatrists judges sleep by how you feel in the day.¹ See how to improve sleep quality for habits that apply to the whole night.
• If you are short of sleep overall, consider the bigger picture before any single stage. Sleep debt explained covers what building up missed sleep means.
• Do not chase a REM number. Tracker estimates are rough,¹³¹⁴¹⁵ and the College gives no target.¹

If you are using a score that mentions recovery, how the readiness score works explains what feeds into it and why one low night is not a verdict.

Frequently asked questions