Sleep stages explained: light, deep and REM sleep through the night

What are the stages of sleep?

You do not sleep in one flat block. According to the National Heart, Lung, and Blood Institute (NHLBI), you cycle through two phases, rapid eye movement (REM) sleep and non-REM sleep, and the cycle starts over every 80 to 100 minutes.¹ Usually there are four to six cycles a night, and you may wake briefly between them.¹

Non-REM sleep is split into three stages. Stage 1 is the changeover from being awake to being asleep. Stage 2 is light sleep. Stage 3 is deep sleep, also called slow-wave sleep after a pattern that shows up in measurements of brain activity.¹ REM sleep is the fourth stage people usually name, and it is the phase where most dreaming happens.

The names differ between sources. Some apps say four stages: light, deeper, deep and REM. The NHLBI and the National Institute of Neurological Disorders and Stroke (NINDS) describe three non-REM stages plus REM, and this guide follows them.¹³

What happens in each stage?

Light sleep: stages 1 and 2

Stage 1 is brief. NINDS describes it as a short period of relatively light sleep in which heartbeat, breathing and eye movements slow, and it usually lasts several minutes.³ Stage 2 follows, and it is the stage where most of a night is spent. Body temperature drops, eye movements stop and brain waves slow, with short bursts of electrical activity.³

Because stage 2 takes the largest share of the night, a night with plenty of it is not a bad night. It is how a normal night is built.

Deep sleep: stage 3

Stage 3 is the deep, slow-wave stage. NINDS says it is the sleep you need to feel refreshed in the morning, and it occurs in longer periods during the first half of the night.³ The NHLBI adds that people usually spend more time in this stage early in the night.¹ Because deep sleep comes early and REM periods get longer later, a shortened night mostly loses the later, REM-richer part of the cycle sequence.

If you want the longer version, see the guide to deep sleep and how much you need.

REM sleep

During REM sleep the brain is active. The NHLBI says brain activity in REM sleep is similar to waking hours, and that dreaming usually happens in this phase.¹ The muscles normally go limp so you do not act out your dreams.¹ REM periods get longer and deeper as the night goes on.³

The NHLBI also notes that people have less REM sleep in colder temperatures, because the body does not regulate its temperature properly during REM.¹ For more, see REM sleep: what it is and how much you need.

How long does a night of sleep take?

A full cycle runs 80 to 100 minutes.¹ With four to six cycles, a night of seven to nine hours fits the pattern. The NHLBI says experts recommend that adults sleep between 7 and 9 hours a night, and that adults who sleep less than 7 hours may have more health issues than those who sleep 7 or more.²

Read the table as a range, not a target. Nobody wakes up neatly at the end of cycle five. The point is only that a normal night holds several cycles, and that cutting a night short removes cycles from the end, where REM periods are longest.³

How do the stages change with age?

The NHLBI states that patterns and types of sleep change as people mature. Slow-wave sleep peaks in early childhood, drops sharply in the teenage years and continues to decrease through adulthood, and older people may have no slow-wave sleep at all.¹

A meta-analysis in the journal Sleep by Ohayon and colleagues pooled 65 studies covering 3,577 healthy people from age 5 to age 102. In adults, total sleep time, sleep efficiency, the percentage of slow-wave sleep and the percentage of REM sleep all decreased with age, while the time to fall asleep, the percentage of stage 1 and stage 2 sleep, and time awake after sleep onset increased. Only sleep efficiency kept falling after age 60.⁴

In plain terms, a lighter, more broken night is common in older adults. It is not by itself a sign of disease. If your sleep changes suddenly, or you feel unrefreshed for weeks, talk to a health professional rather than reading a stage chart.

What can a wrist band tell you about sleep stages?

A sleep study classifies stages from sensors that record brain activity and eye movements.¹ A consumer wellness band does not record either. It senses movement, heart rate, heart rate variability and sometimes skin temperature or blood oxygen, then estimates when you were asleep and how restless the night was. Any stage label a band shows is an estimate, not a sleep-study result.

That makes a band useful for trends. Did you go to bed later this week? Did resting heart rate run higher on nights you slept poorly? Did your sleep time drop below seven hours several nights in a row? Those are questions a band can help you notice. It cannot tell you whether you have a sleep disorder.

For a hands-on look at the signals a band can use, see the guide to heart rate variability and the one on normal resting heart rate.

Habits that protect a normal sleep structure

NINDS notes that sleep affects almost every system in the body, and that chronic lack of sleep or poor quality sleep raises the risk of health problems such as high blood pressure, cardiovascular disease, diabetes, depression and obesity.³ Protecting the structure of a night mostly means protecting its length and its regularity.

• Keep a steady window for sleep so the later cycles, where REM periods are longest, are not cut short.
• Aim for the 7 to 9 hours advised for adults.²
• Remember that caffeine works by blocking adenosine, the chemical that makes you feel sleepy.³
• Keep the bedroom comfortable. The NHLBI notes the body does not regulate temperature well during REM, and that REM is reduced in colder temperatures.¹

None of this replaces medical advice. Loud snoring with pauses in breathing, acting out dreams, or severe daytime sleepiness are reasons to see a clinician, not to adjust a gadget.

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