You open the app in the morning, and it says you had 48 minutes of deep sleep. Is that too little? Is it a lot? And what exactly is REM, which goes up and down every night even though you haven’t changed a thing?
In this article, we explain what happens in each stage of sleep, how long each one lasts, how they change over the years, and which indicators experts use to determine whether sleep is good quality. We’ll also explain what hardly anyone tells you about wristbands: how far what they measure can really be trusted.
| Stage | What happens in your body | Share of the night in an adult | When it occurs most |
|---|---|---|---|
| N1, drowsiness | A transition lasting a few minutes between wakefulness and sleep. Any noise wakes you up | 4-5 % | As you fall asleep and after each awakening |
| N2, light sleep | Muscles relax, and body temperature, blood pressure, and heart rate drop | 45-55 % | All night, and accounts for more of the second half |
| N3, deep sleep | The brain generates slow, wide waves, and you respond less and less to what’s happening around you | 16-20 % | Mostly during the first cycles |
| REM or paradoxical sleep | Brain activity similar to wakefulness, rapidly moving eyes, muscles with no tone, and vivid dreams | 20-25 % | Gets longer toward the end of the night |
Inserm, Sleep dossier, updated in November 2024. These are approximate ranges for healthy adults measured with polysomnography, a sleep laboratory test. The 2017 National Sleep Foundation consensus considers 16 to 20% N3 and 21 to 30% REM typical of good-quality sleep in adults.
What are the stages of sleep?
Sleep has two main parts. Non-REM sleep, which is divided into three increasingly deep stages, and REM sleep, which is something else entirely. It isn’t even deeper sleep; it’s almost its own state.
If you’ve read somewhere that there are five stages and somewhere else that there are four, you’re not going crazy. For decades, laboratories used the 1968 Rechtschaffen and Kales classification, which divided deep sleep into two stages, 3 and 4. In 2007, the American Academy of Sleep Medicine combined them into one, N3. A study that scored the same nights using both systems found that the change significantly shifted the percentages of light and deep sleep, and that these differences varied with age. That’s why the figures in older books and those used today don’t always match.
N1: drowsiness
It’s the gateway. It lasts a few minutes, the muscles begin to relax, and brain activity slows down. Sleep is so fragile at this stage that if someone wakes you, it’s easy to swear you weren’t asleep.
It takes up very little time, around 4 or 5% of the night. And here’s a useful detail. The National Sleep Foundation consensus considers N1 of up to 5% a sign of good quality in adults, and anything above 20% a sign that it isn’t. A lot of N1 usually means you’re falling asleep and waking up repeatedly—in other words, the night is fragmented.
N2: light sleep
It’s the phase you spend the most time in, close to half the night. Body temperature, blood pressure, and heart rate drop, and the muscles relax.
It has a bad reputation because of its name, but if an app tells you that you spent four hours in light sleep, it’s no big deal. That’s normal. The same panel of experts only flagged N2 above 81% of the night as a sign of poor quality.
N3: deep sleep
It is slow-wave sleep. The brain generates large, slow waves, and you become less and less responsive to outside stimuli. That’s why it’s so hard to wake someone during this phase, and why, when you do, it takes them a while to figure out where they are.
It is concentrated in the first cycles of the night, and the body protects it. According to Inserm, the French medical research institute, after a bad night, slow waves are more pronounced, as if the body first recovered what it needs most. It also notes that the amount of deep sleep is relatively constant among people who sleep a lot and those who sleep little. The biggest difference between them is in light sleep.
REM: paradoxical sleep
It was described in 1953 by Eugene Aserinsky and Nathaniel Kleitman in the journal Science, after they observed regular periods of rapid eye movements during the night. Hence the name, rapid eye movement.
It’s also called paradoxical sleep because it’s a contradiction. The brain is almost as active as when awake, but the body’s muscles completely lose their tone. Breathing and blood pressure fluctuate, and this is the phase most conducive to vivid dreams. Inserm attributes an important role to it in emotional memory.
REM sleep lengthens as the night progresses. The first period is short, while the early-morning periods are the longest.
Sleep cycles: how long they last and how many you have each night
The stages don’t just follow one after another. They’re grouped into sleep cycles. In a typical cycle, you go from drowsiness to light sleep, then to deep sleep, back to lighter sleep, and end with REM sleep. Then another cycle begins.
The figure you see repeated everywhere is 90 minutes, and it’s correct as an average. But it’s only an average. Inserm says there are between 3 and 6 cycles per night, each lasting 60 to 120 minutes. They vary from person to person and also from one cycle to another within the same night.
The beginning and end of the night aren’t the same
This is probably the most practical point in the whole article. The first cycles consist mostly of deep sleep. The end of the night, by contrast, is when light and REM sleep take over.
This has a direct consequence. If you cut your sleep short one day by getting up much earlier than usual, what you lose is mostly light and REM sleep, because you’ve already had your deep sleep at the beginning of the night. And if you go to bed very late but get up at the same time, your body tends to prioritize deep sleep in the first few hours, although the total amount of sleep suffers just the same.
It’s normal to have brief awakenings between cycles that you don’t remember in the morning. The National Sleep Foundation panel considers an awakening lasting more than five minutes per night compatible with good-quality sleep, and for people over 65, two awakenings. Waking up once at three in the morning and going back to sleep isn’t a problem.
What alcohol does to your sleep stages
Here’s a perfect example of why “more deep sleep” doesn’t always mean “a better night.” A review of all studies on alcohol and sleep in healthy people found that alcohol, at any dose, makes you fall asleep sooner and consolidates the first half of the night, with more deep sleep at the beginning. But in return, it fragments the second half and delays the onset of the first REM period, which is the most noticeable effect. At moderate and high doses, it also reduces the total percentage of REM sleep.
So the glass of wine with dinner can give you a great first part of the night in the app and a wakeful early morning. If you’ve ever slept terribly after a wedding, you know why.
Is it useful to calculate your wake-up time based on 90-minute cycles?
Some calculators tell you that if you go to bed at 11:30 p.m., you should wake up at 6:45 a.m. to come to the end of a cycle. The idea makes sense, but it is based on an average. If your cycles last 70 minutes or 110 minutes, after five cycles the difference between one person and another is more than three hours. And that's without counting the minutes it takes you to fall asleep.
What does exist is sleep inertia, that grogginess during the first few minutes after waking. A 2019 review on the topic includes studies in which waking from deep sleep worsened immediate performance. In one of them, involving naps, the decline reached 41%, while those who woke from N2 performed as if they had not slept. But the same review cites other studies that find no such relationship and concludes that it is difficult to draw a clear conclusion.
The most strongly supported advice is more boring and works better than any calculator: get up at the same time every day, including weekends. If Mondays make you feel like death, read our article on social jet lag and why getting up early on Mondays feels so hard.
How sleep stages change with age
The sleep stages of a baby, a teenager, and a 70-year-old are quite different. Here is what studies using polysomnography and expert consensus say.
| Stage of life | What changes in the stages | Reference data |
|---|---|---|
| Newborns | Active sleep, the equivalent of REM, takes up a huge part of the night | REM of 41% or more is considered good quality at this age |
| School-age children and adolescents | This is the stage with the most deep sleep, which remains very intense until around age 20 | N3 at 20 to 25% as a benchmark for good quality |
| Adults aged 26 to 64 | The distribution stabilizes | N3 at 16 to 20% and REM at 21 to 30% |
| Every decade in adulthood | You sleep a little less and wake up more | 10 minutes less total sleep, 9.7 more minutes awake after falling asleep, and 2.1 points lower efficiency |
| From age 60-65 | What continues to clearly worsen is sleep continuity | Two awakenings of more than five minutes per night are still compatible with good quality |
National Sleep Foundation, sleep quality recommendations (Ohayon et al., Sleep Health, 2017). Boulos et al., Lancet Respiratory Medicine, 2019, meta-analysis of 169 studies with 5,273 healthy adults. Inserm, Sleep dossier, 2024.
And now the nuance that is almost never mentioned. Everyone takes it for granted that deep sleep decreases with age, and that’s partly true. A 2004 meta-analysis of 65 studies and 3,577 people between the ages of 5 and 102 found that in adults, the percentage of deep sleep and REM decreases with age, while N1, N2, and time awake increase. But it also found something curious: after age 60, the only thing that continued to worsen significantly was sleep efficiency.
Fifteen years later, another much larger meta-analysis, using only studies scored with the current criteria, found that N2, N3, and REM percentages did not change significantly per decade. What did change clearly was everything related to continuity: less total sleep, more time awake, more micro-awakenings, and slightly more N1.
Who is right? Probably both, because they use different criteria. We’ve already seen that the change in criteria in 2007 shifted deep sleep percentages differently depending on age. In addition, the authors of the 2004 study warn that the effect of age varied greatly depending on how well participants were screened for illnesses, medications, alcohol, or apnea.
How to know if you sleep well
This is the question that really matters, and the answer may surprise you. When the National Sleep Foundation brought together a panel of experts to define good-quality sleep, they reviewed 277 studies and voted in three rounds. They reached a clear consensus on indicators of sleep continuity. There was much less agreement on the stages.
In other words, to know whether you sleep well, how continuously you sleep matters more than how many minutes you spend in each stage.
| Indicator | Sign of good quality | Sign that something is wrong |
|---|---|---|
| Time it takes you to fall asleep | Up to 30 minutes | More than 60 minutes (46 to 60 also, except in older adults) |
| Awakenings lasting more than 5 minutes | None or one. Two for those over 65 | Four or more |
| Time awake after falling asleep | 20 minutes or less | 41 minutes or more in adults |
| Sleep efficiency (time asleep divided by time in bed) | 85% or more | 74% or less |
| Deep sleep (N3) in adults | 16-20 % | 5% or less |
| REM in adults | 21-30 % | 41% or more |
Ohayon M, Wickwire EM, Hirshkowitz M, et al. National Sleep Foundation's sleep quality recommendations, first report. Sleep Health, 2017. Expert panel using the Delphi method. These are quality indicators for healthy people, not diagnostic criteria.
A quick example to calculate your sleep efficiency. If you spend 8 hours in bed and sleep for 7, your efficiency is 87.5%. If you spend 9 hours in bed to get those same 7 hours of sleep, it drops to 78%. That’s why spending longer in bed to «catch up» often makes things worse instead of better.
Now for the other half of the answer, from Spanish neurologists. According to the Spanish Society of Neurology, 54% of adults in Spain sleep fewer hours than recommended, 48% don’t get quality sleep, and one in three wake up feeling they haven’t rested. And in 2025, the coordinator of its sleep study group, Dr. Celia García Malo, summed it up this way: the most important marker of your sleep quality «shouldn’t depend on what any kind of electronic device tells us». It’s how satisfied and rested you feel the next morning.
As for how much sleep you need, the consensus of the American Academy of Sleep Medicine and the Sleep Research Society recommends that adults regularly get seven hours or more. And there’s a point worth knowing: many people don’t sleep as much as they think they do, in either direction. We explain this with data in Do you sleep as little as you think?
When to seek medical advice
One bad night is nothing to worry about. But there are situations worth discussing with your primary care doctor, who can refer you to a sleep clinic if needed. If you’ve had trouble falling asleep or staying asleep for more than three months. If you snore loudly and someone has noticed that you stop breathing. Or if you’re so sleepy during the day that you fall asleep on the sofa, in meetings, or while driving. No wristband or app can rule these things out.
What a wristband can and can’t measure about your sleep stages
Sleep stages are defined by brain activity. To see them accurately, a polysomnography is needed. It records brain waves, eye movements, and muscle tone with electrodes while you sleep in a lab. A wristband doesn’t measure any of that. It measures movement, pulse, the variability between heartbeats, and skin temperature, then an algorithm estimates which stage you’re in.
Before judging bracelets, here’s one fact that puts things in context. Even human technicians don’t always agree. In the American Academy of Sleep Medicine’s reliability program, where more than 2,500 professionals scored the same recordings, average agreement was 82.6%. For deep sleep, it fell to 67.4%, and for N1 to 63%. Distinguishing N2 from N3 proved especially difficult even for experts with years of experience.
With that in mind, here’s what validation studies against polysomnography show:
- They’re good at telling whether you’re asleep or not. In a study of 34 healthy young adults over three nights, consumer devices detected sleep with a sensitivity of 93% or higher, and most matched or outperformed traditional actigraphy.
- They have more trouble detecting when you’re awake in bed. Specificity, or correctly identifying periods when you’re awake, ranged from 18 to 54%.
- Sleep stages, inconsistently measured. In the same study, all devices deviated in measuring light sleep, almost always overestimating it, and half overestimated deep sleep or underestimated REM. They also performed worse on bad nights.
- Big differences between devices. A 2023 study of 75 people seen at two sleep centers in Korea compared eleven consumer devices with polysomnography. Agreement on sleep-stage classification, measured on a scale from 0 to 1, was 0.69 for the best device and 0.26 for the worst.
A review of consumer sleep technology offers another warning. Algorithms are proprietary to each manufacturer and are not published, and a software update can change how your figures are calculated from one day to the next.
So, what’s the point of measuring?
What a bracelet really does well: track trends. How much you sleep on average, what time you go to bed and get up, whether your sleep is fragmented on days when you eat dinner late or drink alcohol, and how long it takes you to fall asleep. And, with the sleep stages, to see whether your pattern changes from your normal one in a given week, not to compare your 48 minutes of deep sleep with anyone else’s.
That's the idea behind the Wellbeinn Band. It's a screen-free band you wear day and night that tracks your sleep and its stages—deep, REM, light, and awake—as well as how long it takes you to fall asleep and your sleep efficiency. Overnight, it also estimates your respiratory rate and skin temperature trends, and reads your heart rate and heart rate variability. The Wellbeinn Mate app doesn't compare you to the average person: it needs a few days to learn what's normal for you, and then tells you how you're doing relative to yourself.
Wellbeinn Band
Screen-free band that tracks your sleep and its stages, your heart rate, heart rate variability, skin temperature, and activity. Up to 10 days of battery life. Wellbeinn Mate app included, no subscriptions. This is a wellness device: its metrics are for guidance only.
138,25 €
View Wellbeinn BandIf you also want to track your body composition in the same app, the Band and scale are together in the measurable health collection. And if you're interested in how all this fits with the habits that help you live longer, we cover it in how to live longer and feel better.
Four ideas about sleep stages worth clarifying
“The more deep sleep, the better”
Not necessarily. The expert consensus gives a range for good quality, not “the more, the better.” Alcohol makes this clear: it increases deep sleep in the first half of the night, but fragments the second half and reduces REM sleep. A night with more N3 can be a worse night.
“Waking up at night means you're sleeping poorly”
Not quite. Brief awakenings between cycles are normal, and most aren't remembered. The problem starts when they're long or frequent, or you can't get back to sleep, and when you feel the effects the next day.
“During deep sleep, the brain cleans itself of toxins”
It's one of the most repeated claims, and the evidence is less conclusive than it seems. It comes from a 2013 study in mice that found that the space between neurons increased by 60% during sleep, and beta-amyloid was cleared more quickly. But in 2024, another team, also using mice but a different technique, found just the opposite: the clearance of substances from the brain slowed during sleep and anesthesia. The debate is still open, and so far, neither study has been conducted in humans.
“Older adults need less sleep”
They sleep for shorter stretches, which isn’t the same thing. The National Sleep Foundation recommends 7 to 8 hours for adults over 65, just one hour less at the upper end than for other adults. What changes with age is mainly how easily you stay asleep. If you want to maintain your nighttime routine without pills, we explain the effects of light on rest in red light for sleep, and what we really know about supplements in magnesium and melatonin together.
The essentials in five lines
Frequently asked questions
What are the 4 stages of sleep?
They are N1, N2, N3, and REM. N1 is the process of falling asleep, a transition lasting a few minutes between being awake and asleep that accounts for around 4 or 5% of the night. N2 is light sleep, the longest phase, taking up about half the night. N3 is deep or slow-wave sleep, accounting for 16 to 20% in adults, and is concentrated in the first cycles. REM is paradoxical sleep, when the brain is almost as active as when awake, the muscles have no tone, and dreams are most vivid, accounting for 20 to 25%. If you read somewhere that there are five phases, that’s because deep sleep was divided into two phases, 3 and 4, until 2007, when the American Academy of Sleep Medicine combined them into one.
How long does a sleep cycle last?
On average, about 90 minutes, but that’s an average, not a fixed rule. The French Inserm puts the duration of each cycle at between 60 and 120 minutes, and it varies from person to person and even over the course of the same night. A typical cycle moves from falling asleep to light sleep, then deep sleep, back to lighter sleep, and ends in REM. The first cycles of the night have a lot of deep sleep and a short REM period. The early-morning cycles have little deep sleep and longer REM periods. That’s why it doesn’t make much sense to talk about a “typical cycle” that repeats the same way all night.
How many sleep cycles should you get each night?
Adults usually have 3 to 6 cycles, depending on the length of each one and how long they sleep. With seven and a half hours of sleep and 90-minute cycles, that works out to five, but there’s no specific number you need to reach. The American Academy of Sleep Medicine and the Sleep Research Society recommend getting seven or more hours of sleep regularly, and they don’t talk about cycles. If you get enough sleep, the cycles complete on their own. Nor do they all need to be complete for you to rest: deep sleep, which is the stage the body prioritizes, is concentrated in the first cycles.
How much deep sleep is normal?
In healthy adults, 16 to 20% of sleep time, according to Inserm and the National Sleep Foundation consensus. In a seven-and-a-half-hour night, that’s approximately 72 to 90 minutes. The same panel of experts considers 5% or less to be a sign of poor quality. The percentage is higher in children and teenagers, around 20-25%. Two things to keep in mind. First, the figure given by a fitness tracker is an estimate, and in validation studies, deep sleep is one of the stages that varies most compared with lab measurements. Second, more isn’t always better: alcohol, for example, increases deep sleep during the first half of the night and worsens it during the second half.
How much REM sleep is normal?
In adults, REM accounts for 20 to 25% of the night, according to Inserm, and the National Sleep Foundation consensus considers 21 to 30% good quality. In a seven-and-a-half-hour night, that’s roughly an hour and a half to two hours. REM is concentrated in the second half of the night, so if you don’t sleep much because you get up very early, it’s the stage that suffers most. In adults, REM above 41% isn’t considered a sign of good quality. In newborns, by contrast, active sleep, its equivalent, accounts for a huge part of the night, and 41% or more is expected.
Which sleep stage is the most restorative?
People often say deep sleep, and they’re partly right: it’s the stage the body prioritizes, it’s concentrated at the beginning of the night, and according to Inserm, after a bad night, slow waves are larger, as if the body were making up for it first. But there isn’t just one restorative stage. REM plays an important role in emotional memory, and light sleep accounts for half the night in any healthy person. What makes sleep restorative is that it’s sufficient, continuous, and regular. In fact, when the National Sleep Foundation defined quality indicators, the consensus was clear on continuity and much less so on sleep stages.
What sleep stage do you dream in?
The most vivid, lengthy, and story-like dreams are associated mainly with REM sleep, the phase when the brain’s activity resembles that of wakefulness. During REM, the muscles lose their tone completely, preventing you from physically acting out what you’re dreaming. Because REM periods get longer toward the end of the night, it’s common to remember dreams when you wake up in the early morning or to an alarm. If someone tells you that you shout, hit out, or move a lot while dreaming, mention it to your doctor, as this isn’t expected during this phase.
Why do I wake up tired even after sleeping eight hours?
There may be several explanations. The most harmless is sleep inertia, that grogginess during the first few minutes, which is worse if you have a cumulative sleep deficit or wake up at an unusual time for your body clock. Another possibility is that your eight hours in bed aren’t eight hours of sleep because you spend some time awake without realizing it. Alcohol also plays a role, fragmenting the second half of the night, as do irregular schedules between weekdays and weekends. If this happens almost every day, you snore loudly, or you unintentionally fall asleep during the day, talk to your primary care doctor, as there may be an underlying sleep disorder.
Does a sleep cycle calculator help you wake up feeling better?
It may be useful as a guide, but don’t expect it to be precise. These calculators assume exact 90-minute cycles, when in reality they last 60 to 120 minutes and change throughout the night. After five cycles, the error can be hours. Also, the idea that waking from deep sleep leaves you feeling worse is supported by mixed evidence: a 2019 review on sleep inertia cites studies that confirm this and others that find no connection, and concludes that it’s difficult to draw a clear conclusion. What is well supported is keeping the same wake-up time every day. With a regular schedule, your body anticipates waking up, making early mornings much easier.
Is it normal to wake up several times during the night?
Waking briefly between cycles is normal, and most of these awakenings aren’t even remembered. According to the National Sleep Foundation consensus, one or no awakenings lasting more than five minutes per night is compatible with good-quality sleep, and for people over 65, two is as well. Four or more is not. As we age, the time spent awake during the night also increases: a 2019 meta-analysis calculated nearly ten extra minutes per decade in healthy adults. What’s worth discussing with a doctor is being unable to fall back asleep, having this happen often for more than three months, or noticing the effects the next day in the form of fatigue and irritability.
Why do I get less deep sleep than I did a few years ago?
It could simply be age, although the evidence is less clear-cut than people often say. A 2004 meta-analysis found that the percentage of deep sleep decreases with age in adults. A larger 2019 meta-analysis using current scoring criteria found no significant changes per decade in the percentage of N3 sleep, but did find more time awake, less total sleep, and more brief awakenings. Alcohol, schedules, stress, and health conditions also play a role. And if you’re seeing this on a wristband, remember that deep sleep is one of the stages these devices estimate least accurately. We’ll devote a separate article to this stage if you’d like to learn more.
Are wristbands reliable for measuring sleep stages?
It’s fairly good at telling whether you’re asleep or not. As for sleep stages, treat them as a guide only. A wristband doesn’t record brain activity, which is what defines sleep stages; instead, it tracks movement, heart rate, heart rate variability, and temperature, and an algorithm estimates the stage. In a study comparing devices with polysomnography in 34 healthy adults, the devices detected sleep with at least 93% sensitivity, but their sleep-stage estimates were inconsistent and less accurate on bad nights. For context, two human technicians scoring the same recording agree about 83% of the time, and only 67% of the time for deep sleep. Use it to track trends and compare yourself with yourself, not to judge a single night.
What sleep stages does the Wellbeinn Band show?
The Wellbeinn Mate app shows you your deep, REM, and light sleep and time spent awake each night, as well as how long it took you to fall asleep and your sleep efficiency. Like any wristband, it estimates sleep stages based on signals read from your wrist; it does not measure brain activity. That’s why the app doesn’t compare you with an average: it needs a few days to learn what’s normal for you, and the figures become more accurate over the following weeks. It’s a wellness device, not a medical device. It does not diagnose or rule out sleep disorders, but you can export your records and show them to a healthcare professional if needed.
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