Sleep cycles explained: deep sleep, REM and what your tracker is really telling you
A night's sleep isn't one long stretch. It's a loop of light sleep, deep sleep and REM (dream) sleep that repeats about four to six times, with each loop taking roughly 90 minutes (Sleep Foundation; NHLBI). Deep sleep is packed into the first half of the night and REM sleep gets longer towards morning. There's no single "right" amount of deep sleep, and the stages on a watch or ring are estimates, not a sleep study.
If you've woken feeling flat after what looked like a full night, or stared at your tracker wondering whether your deep sleep is "enough", this guide walks through one night stage by stage. It explains what each stage does, what tends to change in midlife, why dreams can suddenly feel so vivid, and what the numbers on your wrist can and can't tell you.
What are the stages of sleep?
There are four stages of sleep: three stages of non-REM sleep (N1, N2 and N3) and REM sleep (Sleep Foundation). REM stands for rapid eye movement, because your eyes dart about under your eyelids during it.
Here's what each stage does, in plain English.
| Stage | What most trackers call it | What's happening | Share of a typical adult night |
|---|---|---|---|
| N1 | Light | The doorway into sleep. You're easy to wake and might not feel you've slept at all. Lasts 1 to 7 minutes | About 5% |
| N2 | Light (Apple Watch calls it "Core") | Your body temperature drops, muscles relax, and your heart rate and breathing slow | About half the night |
| N3 | Deep | The hardest stage to wake from. Breathing, pulse and muscle tone all drop | Roughly 10% to 25%, depending on the source |
| REM | REM | Your brain becomes nearly as active as when you're awake, most vivid dreams happen, and your muscles go limp | About 25% |
Sources: stage descriptions and timings from the Sleep Foundation; shares from Cleveland Clinic and the Sleep Foundation.
Light sleep isn't wasted sleep. It's about half your night, and it's where your body winds down for the deeper stages.
How long is a sleep cycle?
A sleep cycle usually lasts 80 to 100 minutes, and most people have four to six of them a night (NHLBI). The Sleep Foundation puts the average at about 90 minutes, with the first cycle often a little shorter and later ones a little longer (Sleep Foundation).
The cycles aren't copies of each other. What changes is the mix inside each loop, and that's the key to understanding your night.
One night, stage by stage
Picture your night as a series of waves. The early waves dip deep. The later waves stay closer to the surface and carry longer stretches of dreaming.
The first cycle: dropping in
You drift through light sleep (N1, then N2) and into your first long stretch of deep sleep. In the early cycles, deep sleep can last 20 to 40 minutes at a time (Sleep Foundation). Then you rise back up for a short REM period, often only around 10 minutes (Cleveland Clinic).
The first half of the night: deep sleep's shift
Deep sleep does most of its work in the first half of the night (Sleep Foundation). This is when your body does much of its repair work and releases growth hormone, and it's thought to matter for memory too (Sleep Foundation).
It's also why a late night can feel so costly. If you go to bed much later than usual, it's the deep-sleep-heavy early cycles you're shortening.
The middle of the night: lighter cycles
As the night goes on, the deep stretches get shorter. More of each cycle is light sleep and REM. Because you're sleeping more lightly, it takes less to wake you: a full bladder, a warm room, a partner turning over. That's one reason the early hours are such a common time to find yourself wide awake (read more in Perimenopause and sleep: what changes and what can help).
Towards morning: REM takes over
REM periods get longer in the second half of the night, and the later ones can last around an hour (Sleep Foundation). The Sleep Health Foundation notes that REM comes in short episodes about 90 minutes apart and that "our longer dreams are in the morning hours" (Sleep Health Foundation).
Waking up
Most people surface briefly between cycles, often without remembering it. At the end of the night, you usually wake from light sleep or REM, which is why you sometimes wake mid-dream.
| Part of the night | What dominates | What it feels like |
|---|---|---|
| First cycles | Deep sleep | Hard to wake; groggy if you're woken |
| Middle of the night | Light sleep, with more REM | Easier to wake; brief wake-ups are common |
| Last few cycles | Long REM periods | Dreams, often vivid; easy to wake mid-dream |
How much deep sleep do you need?
There's no agreed number. The Sleep Foundation says "there's no consensus on a single, concrete number for how much deep sleep is normal or ideal" (Sleep Foundation).
As a rough guide, adults typically spend 10% to 20% of the night in deep sleep, which works out to about 40 to 110 minutes over seven to nine hours (Sleep Foundation). Cleveland Clinic puts it closer to 25% (Cleveland Clinic). That spread is the point: normal covers a wide range.
What experts do agree on is total sleep. The Sleep Health Foundation says most adults need 7 to 9 hours a night to feel properly refreshed (Sleep Health Foundation). Getting enough total sleep, at regular times, gives every stage the room it needs.
How much REM sleep do you need?
REM makes up around 25% of adult sleep (Sleep Foundation; Cleveland Clinic). Over seven to nine hours, that's roughly one and three-quarter to two and a quarter hours.
Because most REM sits in the last few cycles, cutting your night short at the end (an early alarm, a 4am wake-up you can't get back from) trims REM more than any other stage.
What is REM sleep, and why do we dream?
REM sleep is the stage when your brain is busiest and most of your vivid dreaming happens. Your brain activity rises to near waking levels, your eyes move under your eyelids, and your muscles go limp so you don't act your dreams out (Sleep Foundation; NHLBI).
Scientists don't fully agree on why we dream. What's well described is that sleep is when the brain sorts and files memories and what you've learned (Cleveland Clinic), and that dreams are often linked to real events, usually from the day or two before (Sleep Health Foundation). You can dream in other stages too, but those dreams tend to be less frequent and less intense (Sleep Foundation).
Why are my dreams so vivid lately?
Here's the honest answer first: we couldn't find good research on vivid or strange dreams in perimenopause or menopause. If you've read that your hormones "cause" vivid dreams, that's ahead of the evidence.
What the science does explain is why you might be remembering more dreams.
- Your longest dreams come in the early morning. That's when REM periods are longest (Sleep Health Foundation).
- You remember a dream when you wake during or just after it. Researchers describe a dream being stored in memory only if you're awake while it's still fresh (Blain and colleagues, 2022).
- Midlife often means more waking in the night. Around 40 to 60% of women have sleep disturbance in perimenopause and after menopause (Sleep Health Foundation).
Put those together and the likely explanation is simple. If you're waking more often in the early hours, you're catching more of your dreams on the way out. The dreams may not be any different. You're just there to remember them.
A few other things are worth knowing:
- Alcohol shifts your stages. A drink before bed changes your sleep early in the night, then tends to bring broken, poorer-quality sleep later on (Colrain and colleagues, 2014).
- Busy days show up at night. Since dreams often draw on the previous day or two, a full-on week can make for full-on dreams (Sleep Health Foundation).
- Medicines matter. If your dreams changed around the time you started or stopped a medicine, mention it to your pharmacist or GP.
How do sleep stages change in midlife?
Over adulthood, sleep slowly gets lighter. A major review of 65 sleep-lab studies (3,577 people) found that as adults get older, the share of deep sleep and REM tends to fall, while light sleep and time awake during the night tend to rise (Ohayon and colleagues, 2004).
But menopause itself isn't the deep sleep cliff the internet suggests. In a 10-year study, researchers measured the sleep of 57 women in a sleep lab before the menopause transition and again ten years later. Their measured sleep didn't get worse because of menopause, the changes looked more related to getting older, and their deep sleep went up over the ten years, not down (Kalleinen and colleagues, 2021). A review of the research also found some studies showing more deep sleep in women during and after the transition (Baker and colleagues, 2018).
So why do so many women feel their sleep has gone downhill? The same review notes that sleep-lab measures "do not necessarily reflect" how women rate their own sleep (Baker and colleagues, 2018). Waking more often, waking hot, and lying awake in the early hours all shape how a night feels, even when the stages on paper look fine. Both things can be true at once. For more on how sleep changes in perimenopause, read Perimenopause and sleep: what changes and what can help.
What do the sleep stages on my watch or ring mean?
Your tracker is making an educated guess. Whether you wear an Apple Watch, a Fitbit, a Garmin or an Oura ring, the stages it shows are estimates, not measurements. Most wearables measure your movement with a sensor called an accelerometer, and newer ones add heart rate and oxygen readings (Sleep Health Foundation). From those signals, an algorithm estimates which stage you're in.
The Sleep Health Foundation puts it plainly: "These devices can usually tell when you are asleep, but they are not very good at telling what sleep stage you are in" (Sleep Health Foundation). A lab study testing seven consumer devices against a full sleep study, in healthy young adults, found the same thing: good at spotting sleep, inconsistent on stages, and less accurate on disrupted nights (Chinoy and colleagues, 2021). Disrupted nights, of course, are exactly the ones you're most likely to check.
Tracker words, translated
| What your tracker says | What it means |
|---|---|
| Awake | Time the device thinks you were awake after you first fell asleep, including brief wake-ups you may not remember |
| Light or Core | N1 and N2 sleep. Apple Watch calls this "Core"; Apple says it's "similar to what some sleep trackers may refer to as light sleep" (Apple) |
| Deep | The device's estimate of N3 |
| REM | The device's estimate of REM sleep |
| Sleep score | The device's own summary of the night, built from its estimates. Each brand works it out differently, and it isn't a medical measure |
How to use your tracker well
- Look at trends over weeks, not single nights. One night's deep sleep figure is an estimate on top of an estimate.
- Trust how you feel. If you wake feeling good, a low score doesn't cancel that out.
- Notice if checking makes it worse. The Sleep Health Foundation notes that for some people who already sleep poorly, tracking can lead to worry about not getting "enough" sleep, and that worry can make sleep harder (Sleep Health Foundation). If that's you, try a week with the screen off.
- Use it to start a conversation, not to diagnose. The American Academy of Sleep Medicine says consumer sleep trackers can't be used to diagnose sleep problems, but can be useful to bring along to a clinician (Khosla and colleagues, 2018).
What does "restorative sleep" actually mean?
"Restorative sleep" isn't a stage you can measure. It describes how sleep feels: waking up feeling refreshed. Researchers who reviewed the idea found there's no standard definition, and proposed describing the opposite as "persistently feeling unrefreshed upon awakening" even when you've slept a normal length of time (Stone and colleagues, 2008).
That matters if you've ever slept eight hours and woken thinking "I need three more". The number of hours, and even the stages on your tracker, don't tell the whole story. How often you wake, how long you lie awake and the timing of your night all play a part.
How to give every stage room
You can't choose your stages directly. What you can do is give your body enough time, at regular times, so each cycle can run its course. These habits come from the Sleep Health Foundation:
- Keep regular sleep and wake times, even after a rough night (Sleep Health Foundation).
- Get outside early in the day. Morning light helps set your body clock (Sleep Health Foundation).
- Give caffeine and alcohol a cut-off of at least 4 hours before bed (Sleep Health Foundation).
- Dim the evening. Screens off 1 to 2 hours before bed if you can (Sleep Health Foundation).
- Cover the clock and, if you're still awake after 20 to 30 minutes, get up and sit somewhere dim until you feel sleepy (Sleep Health Foundation).
When to talk to your GP
A tracker can't diagnose a sleep problem, but your GP can help. Talk to them if:
- your sleep has been off for more than a couple of months, or you wake unrefreshed most mornings despite enough time in bed
- someone has noticed you snoring loudly or stopping breathing in your sleep
- someone has noticed you shouting, kicking or seeming to act out your dreams (Sleep Health Foundation)
- you're drowsy when driving, or tiredness is affecting your work or relationships.
Take your tracker data along if you have it. Your GP can refer you for a proper sleep study if needed. If you need health advice after hours, Healthdirect's free line is 1800 022 222.
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Sweet dreams,
Dominique
Keep reading
- Perimenopause and sleep: what changes and what can help
- Natural sleep habits: simple ways to support better sleep
Frequently asked questions
What are the stages of sleep?
There are four: three stages of non-REM sleep (N1 and N2, often called light sleep, and N3, deep sleep) and REM sleep, when most vivid dreaming happens. You move through them in a loop several times a night.
How long is a sleep cycle?
Usually 80 to 100 minutes, around 90 minutes on average. Most people have four to six cycles a night. Early cycles carry more deep sleep, and later cycles carry longer REM periods.
How much deep sleep do you need?
There's no agreed number. Adults typically spend about 10% to 20% of the night in deep sleep, roughly 40 to 110 minutes over seven to nine hours, though some sources put it closer to 25%. Getting 7 to 9 hours of total sleep matters more than chasing a deep sleep figure.
How much REM sleep do you need?
REM makes up around 25% of adult sleep, roughly one and three-quarter to two and a quarter hours over seven to nine hours. Most REM comes in the last few cycles, so a short night trims it most.
What is REM sleep?
REM stands for rapid eye movement. It's the stage when your brain is nearly as active as when you're awake, your eyes move under your eyelids, most vivid dreams happen and your muscles go limp so you don't act your dreams out.
Why am I having vivid dreams in perimenopause?
There isn't good research on dreams in perimenopause yet. The likely explanation is that your longest dreams come in the early morning, and if you're waking more often then, you remember more of them.
What is core sleep on Apple Watch?
Core is Apple's name for light sleep, the clinical stages N1 and N2. It usually makes up about half the night and isn't a sign of poor sleep.
Are sleep trackers accurate for sleep stages?
They're good at telling when you're asleep but much less reliable at telling which stage you're in, especially on disrupted nights. Treat stages and sleep scores as estimates, look at trends over weeks, and see your GP if you're worried.
Does menopause reduce deep sleep?
Not in a simple way. Deep sleep tends to decline slowly with age, but a 10-year sleep-lab study of women through the menopause transition found their measured sleep didn't worsen because of menopause, and their deep sleep went up rather than down. Many women still feel their sleep is worse, often because they wake more.
About the author
Dominique McAdam is co-founder of LifeLab&Co. She leads the brand's strategy and operations.
This article is general information only and isn't a substitute for advice from your doctor or health professional.
Sources
- Sleep Foundation. Stages of Sleep: What Happens in a Normal Sleep Cycle? Updated 25 July 2025, medically reviewed by Abhinav Singh MD. sleepfoundation.org
- National Heart, Lung, and Blood Institute (NIH). How Sleep Works: Sleep Phases and Stages. nhlbi.nih.gov
- Cleveland Clinic. Sleep: What It Is, Why It's Important, Stages, REM and NREM. Updated 19 June 2023. clevelandclinic.org
- Sleep Foundation. Deep Sleep: How Much Do You Need? Updated 16 July 2025. sleepfoundation.org
- Sleep Health Foundation. How much sleep do you really need? sleephealthfoundation.org.au
- Sleep Health Foundation. Dreaming. sleephealthfoundation.org.au
- Blain S, de la Chapelle A, Caclin A, Bidet-Caulet A, Ruby P. Dream recall frequency is associated with attention rather than with working memory abilities. Journal of Sleep Research. 2022;31(5):e13557. PubMed
- Sleep Health Foundation. Menopause and sleep. sleephealthfoundation.org.au
- Colrain IM, Nicholas CL, Baker FC. Alcohol and the sleeping brain. Handbook of Clinical Neurology. 2014;125:415 to 431. PMC
- Ohayon MM, Carskadon MA, Guilleminault C, Vitiello MV. Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals. Sleep. 2004;27(7):1255 to 1273. PubMed
- Kalleinen N, Aittokallio J, Lampio L, et al. Sleep during menopausal transition: a 10-year follow-up. Sleep. 2021;44(6):zsaa283. PubMed
- Baker FC, Lampio L, Saaresranta T, Polo-Kantola P. Sleep and sleep disorders in the menopausal transition. Sleep Medicine Clinics. 2018;13(3):443 to 456. PMC
- Sleep Health Foundation. Sleep tracker technology. sleephealthfoundation.org.au
- Chinoy ED, Cuellar JA, Huwa KE, et al. Performance of seven consumer sleep-tracking devices compared with polysomnography. Sleep. 2021;44(5):zsaa291. PubMed
- Apple. Estimating Sleep Stages from Apple Watch (October 2025). apple.com
- Khosla S, Deak MC, Gault D, et al. Consumer sleep technology: an American Academy of Sleep Medicine position statement. Journal of Clinical Sleep Medicine. 2018;14(5):877 to 880. PMC
- Stone KC, Taylor DJ, McCrae CS, Kalsekar A, Lichstein KL. Nonrestorative sleep. Sleep Medicine Reviews. 2008;12(4):275 to 288. PubMed
- Sleep Health Foundation. Sleep hygiene: good sleep habits. sleephealthfoundation.org.au
- Sleep Health Foundation. Caffeine, food, alcohol, smoking and sleep. sleephealthfoundation.org.au
- Sleep Health Foundation. REM sleep behaviour disorder. sleephealthfoundation.org.au
Jargon, in plain English
| Term | What it means |
|---|---|
| Sleep cycle | One loop through the sleep stages, about 90 minutes long. You have four to six a night |
| Sleep stages | The different kinds of sleep within each cycle: light (N1 and N2), deep (N3) and REM |
| Non-REM (NREM) | Every stage that isn't REM: N1, N2 and N3 |
| N1 | The lightest sleep, the doorway between awake and asleep |
| N2 | Steady light sleep, about half the night. Apple Watch calls it "Core" |
| N3 (deep sleep, slow-wave sleep) | The deepest, hardest-to-wake stage, mostly in the first half of the night |
| REM | Rapid eye movement sleep. Busy brain, vivid dreams, relaxed muscles. Longest towards morning |
| Core sleep | Apple Watch's name for light sleep (N1 and N2) |
| Sleep score | A tracker's own summary of your night. Each brand calculates it differently; it isn't a medical measure |
| Accelerometer | The movement sensor in a watch or ring that most trackers rely on |
| Sleep study (polysomnography) | An overnight test, usually in a sleep clinic, that measures brain waves, breathing and movement. It's the only way to measure stages properly |
| Perimenopause | The years leading up to your final period, when hormones swing and cycles change |
| Menopause | Your final period, confirmed after 12 months without one |
| Restorative sleep | Sleep that leaves you feeling refreshed. It describes how sleep feels, not a stage |
| Body clock | Your internal 24-hour rhythm that times sleepiness and wakefulness |