There are four stages of sleep: three stages of non-REM sleep (N1, N2 and N3, also called deep sleep) and REM (rapid eye movement) sleep. N1 is the short changeover from waking to sleep, N2 is light sleep and takes up about half the night, N3 is the deep, restorative sleep that dominates the first half of the night, and REM is the active, dreaming stage that gets longer toward morning. You pass through all four several times each night.
How much REM and deep sleep does your night hold? The REM sleep calculator estimates the time you spend in each stage from your bedtime and wake time.
What Are The 4 Stages Of Sleep?
Sleep has two basic types, REM and non-REM, and scientists divide non-REM sleep into three stages, each linked to its own brain waves and nerve cell activity (NINDS). Sleep studies use sensors that record eye movements and brain activity to tell the stages apart (NHLBI). Some sources number REM as “stage 4”; older guides also split deep sleep into stages 3 and 4, but deep sleep is now treated as a single stage, N3.
| Stage | Also called | Typical length | Share of the night (adults) | What happens |
|---|---|---|---|---|
| N1 | Stage 1, light sleep | 1 to 7 minutes | Small | Drifting off; heartbeat, breathing and eye movements slow; easy to wake |
| N2 | Stage 2, light sleep | 10 to 25 minutes in the first cycle, longer later | About half | Body temperature drops, eye movements stop, brief bursts of brain activity |
| N3 | Deep sleep, slow-wave sleep, delta sleep | 20 to 40 minutes in early cycles | 10% to 20% | Slowest brain waves, lowest heart rate and breathing, hard to wake |
| REM | Stage 4, REM sleep | A few minutes at first, up to about an hour later | 20% to 25% | Brain nearly as active as when awake, rapid eye movements, vivid dreams, limb muscles paralysed |
Lengths and shares are averages from the Sleep Foundation and its deep sleep guide; they vary from person to person and night to night.
What Happens In Stage 1 (N1) Sleep?
N1 is the changeover from wakefulness to sleep. During this short period of light sleep your heartbeat, breathing and eye movements slow, your muscles relax with occasional twitches, and your brain waves begin to slow from their daytime pattern. It usually lasts several minutes (NINDS). It is easy to wake someone from N1, and if nothing disturbs you, you move on quickly to N2 (Sleep Foundation).
What Happens In Stage 2 (N2) Sleep?
N2 is light sleep before deeper sleep. Your heartbeat and breathing slow further, your muscles relax more, your body temperature drops and your eye movements stop. Brain activity slows but is marked by brief bursts of electrical activity (NINDS), which help you resist being woken by outside noise. Each N2 period can get longer through the night, and in total a person typically spends about half their sleep in N2 (Sleep Foundation).
What Happens In Deep Sleep (N3)?
N3 is the deep sleep you need to feel refreshed in the morning. Your heartbeat and breathing slow to their lowest levels of the night, your muscles are relaxed, brain waves become even slower, and it can be hard to wake you (NINDS). The slow brain waves are why it is also called slow-wave sleep.
During deep sleep your body repairs tissue, muscle and bone, partly via growth hormone, and it supports the immune system, blood sugar control and memory (Sleep Foundation). Waking from deep sleep can leave you foggy for up to an hour (sleep inertia). Sleepwalking and sleep terrors usually start in this stage; see our guide to sleep stages and dreams for what happens in your mind across the night.
What Happens In REM Sleep?
REM sleep first occurs about 90 minutes after you fall asleep. Your eyes move rapidly from side to side behind closed eyelids, brain wave activity becomes closer to that seen when you are awake, breathing becomes faster and irregular, and heart rate and blood pressure rise to near waking levels. Your arm and leg muscles become temporarily paralysed, which stops you acting out your dreams (NINDS). The muscles that control breathing and the eyes are the exceptions (Sleep Foundation).
Inside the brain, the thalamus, which is quiet during most other stages, becomes active and sends the cortex the images and sounds that fill dreams, and the amygdala, involved in processing emotions, becomes increasingly active (NINDS). Most dreaming happens in REM, although some happens in non-REM sleep.
Each cycle ends in lighter sleep, so waking at the end of one usually feels easier. The sleep calculator times your alarm for those points.
How Do Sleep Stages Change Through The Night?
You do not go through the stages once. You cycle through non-REM and REM several times a night, with longer, deeper REM periods later in the sleep session (NINDS). The pattern shifts like this:
- First half of the night: deep sleep dominates. N3 periods are longest in the early cycles, often 20 to 40 minutes (Sleep Foundation).
- Second half of the night: N3 periods get shorter and REM periods get longer. The first REM period may last only a few minutes, while later ones can last around an hour (Sleep Foundation).
- The order is not fixed: in many cycles you go from N3 back to N2 before REM begins (Sleep Foundation).
How long each cycle lasts, and why waking at the end of one matters, is covered in sleep cycle science.
How Do Sleep Stages Change With Age?
The patterns and types of sleep change as people mature. Newborns spend more time in REM sleep. The amount of slow-wave (deep) sleep peaks in early childhood and drops sharply in the teenage years; it continues to fall through adulthood, and older people may have little or no slow-wave sleep at all (NHLBI). As you age you also spend less of your time in REM sleep (NINDS). The decline in deep sleep is a normal part of ageing and levels off in your 70s (Sleep Foundation).
What Disrupts Deep Sleep And REM Sleep?
| Factor | Effect on the stages |
|---|---|
| Short or irregular sleep | Too little total sleep cuts time in every stage, REM most of all; irregular sleep over days can cause an abnormal sleep cycle |
| Alcohol | Reduces REM early in the night, then causes a REM rebound with prolonged REM as it wears off |
| Caffeine, opioids and some medicines | Can alter sleep patterns and reduce deep sleep; some sleep aids and cannabis can throw off REM |
| Sleep apnea and restless legs | Repeated awakenings stop you cycling properly into deep sleep and REM |
| Stress, pain and mental health conditions | Make deep sleep harder to reach; anxiety and depression can disrupt both deep sleep and REM |
| Light and a cold room | Too much light at night disrupts the body clock and REM; you get less REM in colder temperatures |
Sources: Sleep Foundation stages, deep sleep and REM sleep guides; cold and REM from NHLBI.
How Much Deep Sleep And REM Sleep Do You Need?
For most adults, deep sleep makes up 10% to 20% of total sleep time, which is 42 to 84 minutes on a 7-hour night or 48 to 96 minutes on an 8-hour night. There is no agreed single ideal figure, and outside a sleep lab it is hard to know exactly how long you spend in each stage, so how refreshed you feel is a practical guide (Sleep Foundation). REM typically makes up 20% to 25% of the night, about two hours for most adults (Sleep Foundation).
Both stages partly look after themselves. After a period of short sleep you may spend more time in deep sleep (Sleep Foundation), and a night short of REM can trigger more REM the following night, known as REM rebound (Sleep Foundation).
How Can You Get More Deep Sleep?
The most reliable way is to improve your sleep overall; if your sleep is long enough and good quality, you are usually getting plenty of deep sleep (Sleep Foundation). Habits that help:
- Keep a consistent bedtime and wake time, every day.
- Make the bedroom quiet, cool and dark.
- Wind down with a relaxing routine in the evening.
- Cut caffeine in the afternoon and evening.
- Put screens away for 30 minutes or more before bed.
- Try a warm bath before bed. Some research suggests warming the body this way may bring on more slow-wave sleep.
Regular exercise helps too: NINDS suggests at least 30 minutes on most days, but not within a few hours of bedtime (NINDS).
How Can You Get More REM Sleep?
- Get enough total sleep. If you are not sleeping enough, you are likely getting less REM than you need. Budget for 7 to 9 hours on a consistent schedule, which also keeps the body clock that regulates REM steady (Sleep Foundation).
- Do not cut the morning short. REM periods are longest in the second half of the night, so an early alarm removes REM first.
- Avoid too much artificial light at night, from overhead lights, lamps or screens.
- Skip alcohol and cannabis at night, and manage stress where you can.
- Ask a doctor about medicines and sleep problems. Some medicines, including some sleep aids, and sleep disorders such as insomnia and sleep apnea can keep REM low (Sleep Foundation).
See how much of your night is likely REM and deep sleep with the REM sleep calculator, or check how short nights add up with the sleep debt calculator.
When Should You See A Doctor About Your Sleep Stages?
Talk to a doctor if you feel very sleepy during the day despite enough time in bed, if you suspect a sleep disorder such as sleep apnea, or if you act out dreams by shouting, punching or kicking. That last sign can point to REM sleep behavior disorder, in which the normal REM muscle paralysis fails (Sleep Foundation). A sleep study records breathing, oxygen levels, eye and limb movements, heart rate and brain waves through the night, and shows whether you are moving through the stages properly (NINDS). Most sleep disorders can be treated effectively.
Sleep Stage FAQs
Which Stage Of Sleep Is The Most Important?
All of them. Deep sleep and REM get the most attention, but each stage has a role and you need all of them for healthy sleep.
Is There A Stage 4 Of Sleep?
Many sources call REM “stage 4”. Older systems split deep sleep into stages 3 and 4; today deep sleep is one stage, N3.
Which Stage Of Sleep Do You Dream In?
Most dreaming happens in REM sleep, and REM dreams are the most vivid, but some dreaming happens in non-REM stages too.
Why Is Deep Sleep Called Slow-Wave Sleep?
In N3 the brain produces slow, large delta waves, the slowest brain activity of the night, so it is also called slow-wave or delta sleep.
Can A Sleep Tracker Measure My Sleep Stages?
Not precisely. Consumer trackers cannot measure sleep stages perfectly, but they can show trends over time. A sleep study is the way to measure them accurately.
Why Do I Wake Up Groggy?
Waking from deep sleep can cause sleep inertia, a foggy feeling that can last up to an hour. Waking from lighter sleep is usually easier.