Sleep stages explained: NREM, REM and how your breathing changes

Sleep isn't one long, even state. Through the night you cycle through three stages of non-REM (NREM) sleep and a stage of rapid eye movement (REM) sleep, repeating the loop several times. Your breathing changes along the way: it gets shallower as you fall asleep, the muscles that hold your airway open relax, and both effects are strongest in REM sleep. That's part of why sleep apnea often gets worse during REM sleep.

What are the stages of sleep?

The stages of sleep are three stages of NREM sleep and one stage of REM sleep. The National Heart, Lung, and Blood Institute (NHLBI) says sleep studies tell them apart using sensors that record brain activity and eye movements. Here's how each stage looks in a typical adult night.

What happens in each stage of sleep?

NREM stage 1

Stage 1 is the lightest stage of sleep and the doorway into the night. NHLBI describes it as the transition between wakefulness and sleep. Cleveland Clinic says it usually lasts only a few minutes and makes up about 5% of your sleep time.

NREM stage 2

Stage 2 is light sleep, but it's real sleep. NHLBI says that when you reach stage 2, you are asleep. Cleveland Clinic says your brain waves slow down here, with short, powerful bursts of electrical activity that experts think help organize memories. Stage 2 accounts for about 45% of the night, more than any other stage, and you return to it many times.

NREM stage 3

Stage 3 is deep sleep, also called slow-wave sleep after the brain wave pattern that defines it. NHLBI says you usually spend more time in this stage early in the night. Cleveland Clinic says it makes up about 25% of adult sleep, that your body uses it for repair and immune function, and that you need it to wake up feeling rested. It's hard to wake someone from stage 3, and if you do, they may feel groggy for around 30 minutes.

Deep sleep changes with age. NHLBI says slow-wave sleep peaks in early childhood, drops sharply in the teenage years and keeps decreasing through adulthood, and that older people may not have any at all.

REM sleep

REM sleep is when most dreaming happens. NHLBI says your eyes twitch, your brain becomes active in a way that looks similar to waking, and your muscles normally go limp so you don't act out your dreams. Cleveland Clinic says REM makes up about 25% of total sleep.

NHLBI says you usually get more REM sleep later in the night, and less of it in colder temperatures, because your body doesn't regulate its temperature properly during REM. Our guide to the best bedroom temperature for sleep looks at that in more detail.

How do sleep cycles work?

A sleep cycle is one pass through the NREM stages and into REM, and the cycle repeats several times a night. NHLBI says a new cycle starts every 80 to 100 minutes, that there are usually four to six cycles a night, and that you may wake briefly between them. Cleveland Clinic puts a cycle at about 90 to 120 minutes and says most people go through four or five in a full eight hours.

The mix inside each cycle shifts as the night goes on. Deep sleep is front-loaded, so early cycles hold more stage 3. Cleveland Clinic says the first REM period is usually the shortest, at around 10 minutes, and that each one after it gets longer, up to an hour. So the second half of the night is richer in REM. If you often wake at the same time in the early hours, our article on why you wake up at 3am explores the common reasons.

How much sleep you get affects how many full cycles you complete. Our article on how much sleep adults need covers the recommendations.

How does breathing change during sleep?

Breathing during sleep becomes shallower, slightly faster and less forceful than when you're awake, and these changes are strongest in REM. A 2022 review in the Handbook of Clinical Neurology describes how the depth, rate and regularity of breathing all change after the move from wakefulness to sleep.

The classic measurements come from a 1982 study in Thorax of 19 healthy adults. Compared with being awake:

• the amount of air moved per minute was lower in every stage of sleep
• breathing was more rapid and shallow in every stage of sleep
• ventilation was lowest in REM sleep, when the size of each breath fell to 73% of its waking level
• inspiratory drive, a measure of how hard the brain pushes each breath, was lower in REM than in NREM sleep or wakefulness

The authors concluded that ventilation falls during sleep, with the biggest drop in REM sleep.

Why does your airway relax when you sleep?

Your airway relaxes in sleep because the muscles that hold it open get less signal from the brain once you're no longer awake. In a 2007 study in Thorax, researchers measured muscle activity in 10 healthy people. Activity in the genioglossus, a tongue muscle that keeps the airway open, and the tensor palatini, a muscle in the soft palate, both dropped as people moved from waking into sleep. Tensor palatini activity fell further in stable NREM and REM sleep, while genioglossus activity fell further only in REM. The study suggests that being awake, by itself, helps keep these muscles active.

REM brings the deepest relaxation. A 2014 study in Sleep recorded single motor units in the genioglossus, which the authors call the major upper airway dilating muscle. Fewer units were active in REM than in stage 2, and during the phasic part of REM, when the eyes are moving rapidly, the units fired at much slower rates. Activity also dropped abruptly at the start of shallow-breathing episodes in REM. The authors say this REM-related loss of tone may help explain why obstructive sleep apnea often gets worse in this stage.

How you breathe matters too, through the nose or the mouth. Our article on nasal breathing and sleep quality looks at the research.

What is REM-related sleep apnea?

REM-related sleep apnea is obstructive sleep apnea where the breathing pauses happen mainly or only during REM sleep. A 2021 review in Respiratory Investigation says these REM pauses tend to be longer and cause a deeper drop in oxygen than pauses in NREM sleep. The review also notes that how common it is depends on the definition used, that its clinical significance isn't fully settled, and that it may be linked independently with high blood pressure and poor blood sugar control.

If you snore loudly and also gasp, choke or feel very sleepy in the daytime, see our list of sleep apnea warning signs and talk to a doctor. A proper sleep test, at home or in a lab, is how sleep apnea is diagnosed. Our article on the home sleep apnea test explains the at-home option.

What can a sleep tracker tell you about your sleep stages?

A consumer sleep tracker can give you a reasonable picture of when you were asleep, but its sleep stage breakdown is an estimate. A 2021 study in Sleep tested seven consumer devices, four wearables and three bedside devices, against a lab sleep study in 34 healthy young adults over three nights. The devices were good at detecting sleep, with sensitivity of 0.93 or higher, but weaker at spotting wake, with specificity between 0.18 and 0.54. Their sleep stage results were inconsistent, and most did worse on nights with disrupted sleep.

The American Academy of Sleep Medicine (AASM) said in a 2018 position statement that consumer sleep technologies lack validation against the gold-standard lab test and lack FDA clearance, so they can't be used to diagnose or treat sleep disorders. It also said they can help start a useful conversation with a clinician. That's a good way to use one: look at trends over weeks rather than a single night's stage chart, and bring anything worrying to a doctor. Our guide to snoring apps and sleep trackers covers what they can and can't pick up.

Our pick (paid partner): Titan Sleep System

If your nights are disturbed by a stuffy nose or by mouth breathing and snoring, our pick is the Titan Sleep System. Titan says it combines TitanAir Nasal Strips and Titan Mouth Tape as one nightly system. It won't change your sleep stages, and it isn't a treatment for sleep apnea.

Titan says the strips are drug-free external nasal dilators with flexible spring-loaded bands, worn just above the flare of the nostrils, and that they're designed to pair with the tape. Titan describes the tape as a bamboo silk strip and suggests 20 to 30 minutes of daytime practice before the first night. Titan says every batch is third-party tested and certified PFAS-free, and offers a 30-night guarantee. If you're new to using both, our guide to mouth tape and nasal strips together explains how to start.

Only try mouth tape if you can breathe comfortably through your nose with your mouth closed. We don't recommend mouth tape for children.

Read next

• Can a snoring app or sleep tracker tell you what's going on?
• How much sleep do adults need?
• 10 warning signs of sleep apnea you shouldn't ignore

* This content is for informational and educational purposes only and is not medical advice. It is not intended to diagnose, treat, cure or prevent any disease or health condition. Talk to a qualified healthcare provider about your health.