Sleep stages explained: light, deep and REM sleep, and what your tracker chart shows

What are the sleep stages, in plain English?

Sleep is not one long, even state. The Royal College of Psychiatrists describes two main types, REM sleep and non-REM sleep, and says non-REM sleep has three stages.¹ Together, that gives four stages to know about.

Stage 1 is very light sleep, halfway between being awake and asleep. It normally only happens as you fall asleep, or after waking in the night.¹ Stage 2 is light sleep, and you can wake easily from it without feeling confused.¹ Stage 3 is deep sleep, which the College describes as the most unconscious state of everyday life.¹

REM sleep comes and goes through the night and makes up about one fifth of your sleep time. The brain is very active in REM and this is when you dream.¹ The Sleep Charity also links REM sleep to learning, memory and emotional health.²

You may also see the stages written as NREM1, NREM2 and NREM3. The Sleep Charity uses those labels, and calls NREM3 slow wave sleep.² They are the same stages as above, with different names.

The four stages at a glance

Two points from the table are worth holding on to. First, most of a night is light sleep: stages 1 and 2 together come to 48 to 55% on The Sleep Charity’s figures.² Second, deep sleep is the stage hardest to leave, which is why being woken from it feels so groggy.²

How does a night fit together? A worked example

The Royal College of Psychiatrists says each cycle has several stages that usually run in the same order, for example non-REM 1, 2, 3, 2, then REM, and that adult cycles last about 90 minutes each.¹ The Sleep Charity says a good night usually includes four to five full cycles.²

From 23:00 to 06:30

Take someone in bed from 23:00 to 06:30. That is 7 hours 30 minutes, or 450 minutes. Divided by 90 minutes a cycle, 450 ÷ 90 = 5 cycles, which sits inside the four to five the Sleep Charity describes.² The numbers below are an illustration, not a typical night.

For simplicity, treat all 450 minutes as sleep and apply the Sleep Charity’s shares. NREM1 at 3 to 5% is 13.5 to 22.5 minutes. NREM2 at 45 to 50% is 202.5 to 225 minutes. NREM3 at 20 to 25% is 90 to 112.5 minutes, so about 1 hour 30 minutes to 1 hour 52 minutes. REM at about 20% is 90 minutes.¹²

In real life you are not asleep for every minute in bed. Suppose 50 minutes of that time is spent awake. Sleep would then be 400 minutes, and every figure above would fall by about 11%. Deep sleep would be 80 to 100 minutes instead. The 50 minutes is invented to show the arithmetic.

Notice also where the stages sit. The first couple of cycles have a lot of deep sleep and not much light sleep, while later cycles have very little deep sleep and lots of light sleep.¹ A chart that shows deep sleep bunched near the start of the night is therefore what the sources would lead you to expect.

Deep sleep versus REM sleep: what each is linked to

The two stages do different jobs, and none of the sources read ranks one above the other.

• Deep sleep. The Royal College of Psychiatrists says stage 3 is important for memory processing, mental function and the physical repair of the body.¹ It is also the stage that sleep terrors and sleepwalking usually come from.¹
• REM sleep. The Sleep Charity says this is when most dreaming happens, and ties it to learning, memory and emotional health.²

So the question “which matters more?” has no answer in these sources. They describe different roles and leave it there. For a closer look at the deepest stage, see the guide to how much deep sleep you need.

Do sleep stages change with age?

Yes, in direction if not in exact numbers. A meta-analysis of 65 studies covering 3,577 healthy people aged 5 to 102 found that, in adults, the percentage of slow-wave sleep and of REM sleep fell with age, while the percentages of stage 1 and stage 2 and the time awake after falling asleep rose.⁴

The size of those effects depended on how carefully the participants had been screened for disease and sleep disorders.⁴ The abstract read for this guide gives the direction of each change but not percentages for each age group, so this guide does not give any.

The Sleep Charity makes the same point in plainer words. With age, people spend less time in NREM stage 3 and wake more often, while sleep needs stay much the same.⁵

How far can you trust the stages on a wrist tracker?

In the studies read for this guide, the reference for sleep staging is polysomnography (PSG), a laboratory recording, and consumer devices are judged by how well their stages match it.

The evidence read for this guide is cautious. A systematic review of Fitbit models concluded that their sleep stage assessments are of limited specificity and not a substitute for PSG.⁶ It also found that newer models, which use heart rate variability together with body movement, did better than early models that used movement alone.⁶

A laboratory study of seven consumer devices against PSG in 34 healthy young adults reached a similar view. Sleep stage comparisons were mixed, and the authors described the device stage assessments as inconsistent.⁷

The practical reading is modest. Treat the shape of the night, such as roughly when you fell asleep and woke, with more confidence than the exact minutes in each stage.

How to read your stages chart without over-reading it

• Check the time asleep and the time awake first. The Sleep Charity describes good quality sleep as falling asleep within 15 to 20 minutes.³
• Look at the pattern. A lot of deep sleep early on, and mostly light sleep later, is the pattern the College describes.¹
• Compare with your own recent nights rather than with a figure from a guide. Device sleep stage estimates were inconsistent with polysomnography in one study, and worse on disrupted nights.⁷
• Remember that the sources read give no minutes-per-stage targets, so a low deep sleep figure on its own is not a verdict.
• Judge your sleep by how you feel in the day as well as by the chart.

Frequently asked questions