We spend about a third of our lives asleep (NINDS, 2025). But sleep isn't one even state: your brain moves through different stages during the night.
Millions of people now track their sleep with phones, watches and rings (NINDS, 2025). This guide explains the stages in plain English, using the names Apple Health shows, what other trackers call them, and how far you can trust the numbers.
Key insight
Most sleep trackers estimate your sleep stages – they don't measure them directly. Treat the numbers as a guide to patterns over weeks, not a verdict on one night.
The four sleep stages in Apple Health
Scientists divide sleep into REM (rapid eye movement) sleep and three non-REM stages (NINDS, 2025). Apple Health groups these into four stages, and Apple Watch sorts every 30 seconds of your night into one of them (Apple, 2025).
| Apple Health stage | What happens |
|---|---|
| Awake | Time awake during the night. Brief wake-ups are common in normal sleep, and you may not remember them (Apple, 2025). |
| Core | Apple's name for non-REM stages 1 and 2: dozing off and light sleep (Apple, 2025). Your heartbeat and breathing slow and your temperature drops. You spend more time here than in any other stage (NINDS, 2025). |
| Deep | Non-REM stage 3 (Apple, 2025), the deep sleep you need to feel refreshed. Your heartbeat and breathing are at their slowest, and you may be hard to wake (NINDS, 2025). |
| REM | Your eyes move quickly behind closed eyelids and your brain becomes more active. Most dreams happen here, and your arm and leg muscles are temporarily paralysed so you don't act them out (NINDS, 2025). |
What other trackers call each stage
Different brands use slightly different names for the same stages:
| Apple Health | Oura and Fitbit | Sleep science |
|---|---|---|
| Awake | Awake | Wake |
| Core | Light | Non-REM stages 1 and 2 |
| Deep | Deep | Non-REM stage 3 |
| REM | REM | REM |
Apple chose "Core" rather than "light" because this stage is a normal and important part of sleep (Apple, 2025). Oura and Fitbit sort sleep into wake, light, deep and REM (Robbins et al., 2024).
How the stages fit together
You cycle through the stages several times during a typical night (NINDS, 2025). REM sleep first starts about 90 minutes after you fall asleep, and REM periods get longer later in the night. Deep sleep comes in longer stretches during the first half of the night. As you get older, you spend less time in REM sleep (NINDS, 2025).
An illustration of how sleep stages repeat through a typical night.
There's no single "right" amount of each stage. In the adults Apple used to test its watch, full sleep studies found an average of about 66% Core, 21% REM and 13% Deep sleep, but individual nights varied widely (Apple, 2025). On average, adults need 7 to 9 hours of sleep in total. If you're constantly tired during the day, you're probably not getting enough (NHS, 2025a).
Apps and devices you can use
Smart technology can record sounds and movement, hours slept, heartbeat and breathing (NINDS, 2025).
| Type | What it does |
|---|---|
| Smartwatches and fitness bands | Apple Watch estimates REM, Core and Deep sleep and when you might have woken (Apple, n.d.). Fitbit is another popular example (Robbins et al., 2024). |
| Smart rings | Worn on a finger. The Oura Ring also estimates sleep stages (Robbins et al., 2024). |
| Bedside monitors and headbands | Track sleep without a watch (NINDS, 2025). |
| Phone apps | Record sounds and movement (NINDS, 2025). |
| Sleepio (for insomnia) | A six-week digital cognitive behavioural therapy programme with a sleep diary (HTN, 2022). NICE recommends it for adults with insomnia in primary care (NICE, 2022). Ask your GP if it's available in your area. |
| Sleep study (polysomnogram) | The gold standard, arranged by your doctor. It records brain waves, breathing, oxygen levels, heart rate and movements, usually overnight at a sleep centre (NINDS, 2025). |
How accurate are sleep trackers?
In a 2024 study, 35 healthy adults wore an Oura Ring, a Fitbit and an Apple Watch during a night in a sleep lab. All three detected sleep at least 95% of the time. But they were much less reliable at identifying the exact stage, correctly spotting between 50% and 86% of it depending on the device and stage (Robbins et al., 2024). Oura funded the study (Sleep Review, 2024), and the volunteers were healthy adults aged 20 to 50, so results may differ for older people or those with health conditions.
Apple's own testing found deep sleep was most often mislabelled as Core (Apple, 2025). Bear in mind this comes from Apple's report on its own product. Either way, your tracker is good at knowing whether you slept, but its stage breakdown is an educated guess.
Don't let the numbers keep you awake
Sleep doctors have described patients so focused on "perfect" tracker scores that the worry made their sleep worse. They called this orthosomnia (Baron et al., 2017). If checking your data makes you anxious, take a break from it.
Signs to talk to a GP about
A tracker can't diagnose a sleep disorder. See a GP if your breathing stops and starts while you sleep, you make gasping, snorting or choking noises, or you always feel very tired during the day. These can be signs of sleep apnoea, which can be serious if it isn't treated. If someone has seen these symptoms, it can help to bring them with you (NHS, 2025b).
Also see a GP if changing your sleeping habits hasn't helped, you've had trouble sleeping for months, or poor sleep is making it hard to cope. You may be offered cognitive behavioural therapy (CBT), face to face or online (NHS, 2025a).
Feeling sleepy? Don't drive
The NHS advises not to drive when you feel sleepy (NHS, 2025a).
Want to go further?
Our guide to using AI to understand your sleep and health data shows how to combine your tracker data with medicines and lifestyle notes – safely.
More from Zero Jargon Health
Read our posts on sleep hygiene and insomnia.
The bottom line
Each night you cycle through Core, Deep and REM sleep several times, with brief spells awake. Trackers are good at spotting when you sleep, less good at naming each stage. Use them for patterns, not perfection – and talk to a GP about anything that worries you.
References
Apple (2025) Estimating sleep stages from Apple Watch. Updated October. Available at: https://www.apple.com/kr/health/pdf/Estimating_Sleep_Stages_from_Apple_Watch_Oct_2025.pdf (Accessed: 29 September 2026).
Apple (n.d.) Track your sleep with Apple Watch. Apple Watch User Guide. Available at: https://support.apple.com/guide/watch/apd830528336 (Accessed: 29 September 2026).
Baron, K.G., Abbott, S., Jao, N., Manalo, N. and Mullen, R. (2017) 'Orthosomnia: are some patients taking the quantified self too far?', Journal of Clinical Sleep Medicine, 13(2), pp. 351–354. Available at: https://doi.org/10.5664/jcsm.6472 (Accessed: 29 September 2026).
HTN (2022) NICE recommend digital self-help programme Sleepio to treat insomnia. 24 May. Available at: https://htn.co.uk/2022/05/24/nice-recommend-digital-self-help-programme-sleepio-to-treat-insomnia/ (Accessed: 29 September 2026).
National Institute for Health and Care Excellence (2022) Sleepio to treat insomnia and insomnia symptoms (HTG624). Available at: https://www.nice.org.uk/guidance/htg624 (Accessed: 29 September 2026).
NHS (2025a) Insomnia. Last reviewed 12 November. Available at: https://www.nhs.uk/conditions/insomnia/ (Accessed: 29 September 2026).
NHS (2025b) Sleep apnoea. Available at: https://www.nhs.uk/conditions/sleep-apnoea (Accessed: 29 September 2026).
NINDS – National Institute of Neurological Disorders and Stroke (2025) Brain basics: understanding sleep. Last reviewed 25 February. Available at: https://www.ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-understanding-sleep (Accessed: 29 September 2026).
Robbins, R. et al. (2024) 'Accuracy of three commercial wearable devices for sleep tracking in healthy adults', Sensors, 24(20), 6532. Available at: https://doi.org/10.3390/s24206532 (Accessed: 29 September 2026).
Sleep Review (2024) Oura Ring, Apple Watch, and Fitbit tested against PSG in sleep accuracy study. Available at: https://sleepreviewmag.com/sleep-diagnostics/consumer-sleep-tracking/wearable-sleep-trackers/oura-ring-apple-watch-fitbit-face-off-sleep-accuracy-study/ (Accessed: 29 September 2026).
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Important medical disclaimer
The content provided in this blog post is for educational and informational purposes only and does not constitute professional medical advice, diagnosis, or treatment. Always seek the advice of your GP, psychiatrist or another qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. Never start, stop or change a prescribed medicine without speaking to your prescriber first.
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