Two devices, one place
How do wearables measure sleep stages?
Updated · 5 min read
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Short answer
Wearables estimate sleep stages: a model checked against sleep-lab tests labels each part of the night from movement, and often heart rate and breathing.
Apple Watch, Oura and WHOOP do not read the brain waves that a sleep lab records, so each one estimates your sleep stages from body signals. Apple Watch uses wrist movement alone, while Oura and WHOOP add heart rate, breathing rate and heart rate variability (HRV), a measure of how the time between heartbeats varies.
What a sleep lab measures
A sleep lab test is called polysomnography (PSG). The US National Institutes of Health says that it records your breathing, oxygen levels, eye and limb movements, heart rate and brain waves through the night.
Trained technicians then label each 30 seconds of the recording with a stage. The lab uses five stages: awake, REM (rapid eye movement) sleep and three non-REM stages, N1, N2 and N3. The National Institutes of Health says that each of the three non-REM stages is linked to specific brain waves.
Apple gives the lab stage for each of its stage names:
| Lab stage | Apple Watch name |
|---|---|
| N1 and N2 | Core |
| N3 | Deep |
| REM | REM |
Oura and WHOOP use the names Light, Deep and REM. Both call deep sleep slow-wave sleep, which Apple's paper calls N3.
What each wearable reads
| Device | Where you wear it | Signals for stages | Time that each label covers |
|---|---|---|---|
| Apple Watch | Wrist | Movement from the accelerometer, the motion sensor | 30 seconds |
| Oura | Finger | Movement, skin temperature, resting heart rate, HRV and breathing rate | 30 seconds |
| WHOOP | Wrist | Heart rate, HRV, breathing rate and movement | Not given on WHOOP's page |
Apple trained and tested its model on lab and home PSG recordings that experts labeled. WHOOP says that it trained its models on PSG data and WHOOP data. Oura compares its stages with PSG tests.
How your body changes between stages
Your body changes from one stage to the next. The National Institutes of Health says that your heartbeat and breathing slow to their lowest levels in deep sleep. In REM sleep, your breathing becomes faster and irregular, and your heart rate rises to near waking levels.
Oura says that it reads body signals that change as you enter each stage. Apple says that the watch's motion sensor samples your wrist often enough to pick up the movements of breathing.
How close the estimates come to a sleep lab
- Agreement with a sleep lab
Oura says its staging algorithm achieved 79% agreement with PSG sleep-lab tests. Oura describes accuracy as the share of 30-second windows where the ring and a technician give the same label.
- Deep sleep labeled as light sleep
Apple says that the most common error of its model is true deep sleep labeled as Core.
- Wake labeled as sleep
Oura says that the ring is better at recognizing sleep than at recognizing wake. Apple says that its 2025 update better recognizes quiet wake: time when you are awake but very still, often just before or after sleep.
- Stages in general
In a Central Queensland University sleep-lab study of six devices on 53 healthy adults, the authors found that no device was yet adequate for measuring specific sleep stages, as WHOOP reports the study. WHOOP adds that the study tested WHOOP 3.0, and that its hardware and algorithms have changed since.
See your sleep stages
- Open the Health app on your iPhone.
- Tap Browse then Sleep then Sleep.
Step 2
Where to tap
- Browse
- Sleep
- Sleep
- Tap Show More Sleep Data.
- Tap Stages. This view shows the time and percentage that you spent Awake and in REM, Core and Deep sleep.
On Apple Watch, open the Sleep app and turn the Digital Crown to see your stages for the last 14 days. In the Oura app, open the Sleep detail view. Your stages are at the top of the screen, and the hypnogram, a graph of your stages across the night, is under Details.
If your stages look wrong
- No sleep data from Apple Watch
Apple says that you must turn on Track Sleep with Apple Watch, the setting that lets the watch record your sleep, and wear the watch for at least 1 hour each night. If you have not set up Sleep, open the Health app on iPhone and tap Browse then Sleep then Sleep. Tap Get Started below Set Up Sleep, then follow the prompts, which include Track Sleep with Apple Watch. Charge the watch to at least 30% before bed.
- Asleep and awake, but no stages
Apple says that Apple Watch writes the full stages to Apple Health only when you sleep more than three hours in a detected sleep session. When you sleep one to three hours, the watch writes only asleep and awake.
- A loose watch or band
Apple says that a loose watch might register too much movement during sleep, so wear it at a comfortable fit. WHOOP asks you to wear its band snugly, about one inch above your wrist bone.
- No WHOOP stages in Apple Health
WHOOP's list of what it exports to Apple Health says "Sleep tracking". One WHOOP member reported that Apple Health shows only Time in Bed and Time Asleep from WHOOP, while the WHOOP app shows the stages. To see your stages, open the WHOOP app, which shows your light, slow wave (deep) and REM sleep.
- Oura stages look different in Apple Health
Oura says that Apple may apply slight rounding to the data or draw its graphs with different time intervals. To see Oura's own graph, open the Sleep detail view in the Oura app, as in "See your sleep stages" above.
Questions
Can a wearable measure REM sleep?
Not directly. In a sleep lab, REM sleep shows in rapid eye movements, a change in breathing and brain waves that look like waking, Apple says. A wearable gives an estimate instead, from the body signals in the table under 'What each wearable reads'.
What is Core sleep on Apple Watch?
Apple says Core is similar to what some sleep trackers call light sleep. Apple chose Core to avoid the word light, because stage N2 often makes up more than 50% of a night's sleep and is a normal part of sleep.
Does Oura send sleep stages to Apple Health?
Yes. Oura lists Sleep Stages in the data that it exports to Apple Health. On Gen3 and later rings, Oura's Apple Health sharing needs an active membership.
Sources
- 1Estimating Sleep Stages from Apple Watch, Appleapple.com (opens in a new tab)
- 2Track your sleep on Apple Watch and use Sleep on iPhone, Apple Supportsupport.apple.com (opens in a new tab)
- 3Set up sleep schedules on Apple Watch, Apple Supportsupport.apple.com (opens in a new tab)
- 4Sleep Stages, Oura Helpsupport.ouraring.com (opens in a new tab)
- 5How Does the Oura Ring Track My Sleep?, Ouraouraring.com (opens in a new tab)
- 6Apple Health Integration, Oura Helpsupport.ouraring.com (opens in a new tab)
- 7Sleep Accuracy: How WHOOP Measures and Scores Sleep, WHOOPwhoop.com (opens in a new tab)
- 8Apple Health Integration, WHOOP Supportsupport.whoop.com (opens in a new tab)
- 9Apple Health missing sleep stage data from WHOOP, WHOOP Communitycommunity.whoop.com (opens in a new tab)
- 10Monitor your heart rate with Apple Watch, Apple Supportsupport.apple.com (opens in a new tab)
- 11Brain Basics: Understanding Sleep, National Institutes of Healthninds.nih.gov (opens in a new tab)