Quick Answer: Sleep has two phases: non-REM and REM. Non-REM has three stages, N1, N2 and N3 deep sleep, so there are four stages in all, counting REM. NHLBI says the cycle repeats every 80 to 100 minutes, four to six times a night. Deep sleep is more common early in the night, and REM later.
Want to time your wake-up around the cycle? Try the REM sleep calculator.
What Are The 4 Stages Of Sleep?
When you sleep, you cycle through two phases: rapid eye movement (REM) sleep and non-REM sleep. Non-REM sleep has three stages, defined by brain activity measured in sleep studies (NHLBI). Together with REM, that gives four stages: N1, N2, N3 and REM.
Here is what NINDS describes for each one (NINDS).
| Stage | What Happens in the Body | Good to Know |
|---|---|---|
| N1 | Heartbeat, breathing and eye movements slow, and muscles relax with occasional twitches | The changeover from waking to sleep. Usually lasts several minutes |
| N2 | Heartbeat and breathing slow further, body temperature drops and eye movements stop | Light sleep before deeper sleep. You spend more of each night here than in any other stage |
| N3 | Heartbeat and breathing reach their lowest levels, muscles are relaxed and brain waves are even slower | Deep sleep. Longer periods in the first half of the night, and it can be hard to wake you |
| REM | Eyes move rapidly, breathing is faster and irregular, and heart rate and blood pressure rise toward waking levels | Most dreaming happens here. Arm and leg muscles are temporarily paralyzed |
NHLBI calls N3 deep sleep or slow-wave sleep, named for a pattern in brain activity. It describes REM as the stage when your eyes twitch and your brain is active, in a way that resembles waking hours.
How Much Of The Night Does Each Stage Take?
The American Thoracic Society (ATS) primer for reading sleep studies gives typical adult shares. About 5 percent of the night is N1, 50 percent is N2, 20 percent is N3 and 20 to 25 percent is REM (ATS primer).
These are typical amounts, not targets. For minutes and what is normal for you, see how much deep sleep you need.
Sleep Cycle Length: How Long Is One Cycle?
NHLBI says the cycle starts over every 80 to 100 minutes, with usually four to six cycles a night. You may wake up briefly between cycles. NINDS adds that REM sleep first occurs about 90 minutes after you fall asleep.
It also says REM periods get longer and deeper later in the sleep session.
The cards show typical figures, and real nights vary from person to person. To plan a wake-up that lands between cycles, see sleep cycle science.
Which Stages Come Early And Which Come Late?
The mix shifts across the night. NHLBI says you usually spend more time in deep sleep early in the night and have more REM sleep later.
NINDS agrees that N3 occurs in longer periods during the first half of the night, with increasingly longer, deeper REM periods later. This is why cutting a night short can change which stages you miss. NHLBI also says you do not get as much REM sleep in colder temperatures, because your body does not regulate its temperature properly during REM.
How Do Sleep Stages Change With Age?
Patterns change as people mature, NHLBI says. Newborns spend more time in REM sleep. Slow-wave sleep peaks in early childhood and then drops sharply in the teenage years. It keeps decreasing through adulthood, and older people may not have any slow-wave sleep at all. NINDS adds that you spend less of your time in REM sleep as you age.
Because deep sleep declines for everyone, a lower number later in life is common. NINDS adds that older people may have more trouble getting enough sleep and are more likely to take medicines that can interfere with it. See how much sleep older adults need for total sleep by age.
Which Parts Of The Brain Control The Stages?
NINDS says several brain structures are involved. The hypothalamus contains nerve cells that act as control centers for sleep and wakefulness.
Within it, the suprachiasmatic nucleus receives information about light directly from the eyes and sets your daily rhythm. The brainstem controls the transitions between waking and sleep, and it plays a special role in REM sleep. It sends signals that relax your muscles so you do not act out your dreams.
Dreaming And The Stages
Everyone dreams, says NINDS. You spend about two hours each night dreaming, but you may not remember most of it. Dreams can occur in all stages of sleep, though most dreaming happens in REM. People under stress or anxiety are more likely to have frightening dreams. To go deeper, read why do we dream.
How Are Sleep Stages Measured?
NHLBI says sleep studies use sensors to record eye movements and brain activity, and these measurements classify the phases and stages.
This is how stage counts are made in a lab, and it is the basis of the percentages above. Wearables use different methods and give estimates, so read are sleep trackers accurate before trusting a single night.
If you have a study report, how to read sleep study results explains the terms.
What Controls When You Sleep?
NINDS says two internal mechanisms work together: the circadian rhythm and sleep-wake homeostasis. Circadian rhythms follow your body clock and make you sleepy at night. Homeostasis keeps track of your need for sleep, and the sleep drive gets stronger every hour you are awake.
Melatonin helps you feel sleepy when it gets dark, and a chemical called adenosine builds up and makes you sleepy. Light is perhaps the greatest influence on your sleep-wake needs, NINDS says. Caffeine counteracts sleepiness by blocking adenosine, so late caffeine can work against sleep.
NINDS also notes that exposure to light can make it harder to fall asleep, or to get back to sleep if you wake in the night.
How Much Sleep Do You Need?
NINDS says most adults need 7 to 9 hours a night, school-age children and teens about 9.5 hours, and babies initially 16 to 18 hours a day. It adds that there is no magic amount that works for everybody of the same age.
Many people feel they can catch up on missed sleep on weekends, but sleeping longer on weekends may not be enough to replace the sleep they have missed.
Why Does Sleep Matter For Health?
NINDS says everyone needs sleep, though its biological purpose remains a mystery. Without sleep, you cannot form or maintain the brain pathways that let you learn and create new memories, and lack of sleep makes it harder to concentrate and react quickly. Recent findings suggest sleep plays a housekeeping role that clears toxins that build up while you are awake.
The agency also says a chronic lack of sleep, or poor quality sleep, increases the risk of health problems such as high blood pressure, cardiovascular disease, diabetes, depression and obesity.
Which Stage Matters Most?
None can be skipped. NINDS calls N3 the deep sleep you need to feel refreshed in the morning. It also says memory consolidation most likely requires both non-REM and REM sleep. So the goal is enough total sleep, with the stages left to sort themselves out. If you have tracker numbers, remember they are only estimates.
When To See A Doctor
See a doctor if poor sleep lasts for weeks, you are very sleepy in the day, or someone sees you stop breathing or gasp in your sleep. Odd stage numbers from a tracker are not a reason to worry on their own, but a sleep study can measure stages directly.
If you nod off while driving, stop driving and get help soon. A sleep specialist can arrange a study.
FAQs About The Stages Of Sleep
How Many Stages Of Sleep Are There?
Four, counting REM. Non-REM sleep has three stages, N1, N2 and N3, and REM is the fourth. NHLBI describes two phases and three non-REM stages.
Which Stage Is Deep Sleep?
N3. NHLBI calls it deep sleep or slow-wave sleep, and says you usually spend more time in it early in the night.
Do You Only Dream In REM Sleep?
Mostly. NHLBI says dreaming usually happens in REM sleep. NINDS says most dreaming occurs there, though some can occur in non-REM sleep.
When Does REM Sleep Start?
About 90 minutes after you fall asleep, per NINDS. REM periods get longer later in the night.
Does Everyone Dream?
Yes, says NINDS. You spend about two hours each night dreaming, but you may not remember most dreams. Their exact purpose is not known.
Do You Wake Up Between Cycles?
Sometimes. NHLBI says you may wake up briefly between cycles. Whether you remember it varies, and a brief waking is not a problem by itself.
Your Next Step
Enter the deep sleep and REM minutes from your tracker to see the shares of your night. Trackers give estimates only.