Miya Pearson
The Apple Watch uses signals from its accelerometer and heart rate sensor to detect when users are in REM, Core, or Deep sleep. The Sleep app on the Apple Watch allows users to create sleep schedules to help them meet their sleep goals. The watch can estimate the time spent in each sleep stage and when the user woke up. The watch's sleep tracking feature is available on watchOS 8 or later.
| Characteristics | Values |
|---|---|
| How does it work? | Using signals from the accelerometer and heart rate sensor, Apple Watch can detect when users are in REM, Core, or Deep sleep. |
| How accurate is it? | The Apple Watch overestimates total sleep time by an average of 39.5 minutes, with an absolute bias of 48.1 minutes. |
| How does it compare to other wearables? | Unlike other wearables such as the Fitbit or Oura, which measure how much time you spend in the various sleep phases and give calculated sleep quality scores, Apple's sleep tech is more simplified. |
| How does it compare to clinical sleep studies? | Apple's press release states that "The machine learning models were trained and validated against the clinical gold standard, polysomnography, with one of the largest and most diverse populations ever studied for a wearable." |
| How do you set it up? | Make sure your Apple Watch has WatchOS 7 or later and your iPhone has iOS 14 or later. Then, go to the Health app on your iPhone and configure sleep. You can also do this on your Apple Watch. |
What You'll Learn
Apple Watch's sleep tracking features
The Apple Watch's sleep tracking features are designed to help you understand your sleep patterns and improve your sleep quality. Here's a detailed overview of these features:
Sleep Tracking Technology
The Apple Watch uses advanced technology to track your sleep. It combines data from the accelerometer, which detects movement, and the heart rate sensor to identify the different stages of sleep: REM, Core, and Deep sleep. This combination of motion and heart rate data allows the watch to estimate your sleep stages with approximately 80% accuracy.
Sleep Schedules and Goals
With the Sleep app on your Apple Watch, you can create customised sleep schedules. You can set a sleep goal, such as the number of hours of sleep you want to achieve. The app also allows you to set a bedtime reminder and a wind-down period before bed, helping you establish a consistent sleep routine. Additionally, you can enable Sleep Focus, which limits distractions and protects your sleep by turning off the watch display.
Sleep Tracking and Analysis
The Apple Watch automatically tracks your sleep when you wear it to bed. It detects when you fall asleep, and upon waking up, you can view a detailed analysis of your sleep. The watch provides information on the duration of your sleep, the time spent in each sleep stage, and your sleep trends over the past 14 days. This data is synchronised with the Health app on your iPhone, where you can access more in-depth sleep reports, including respiratory rate and heart rate data.
Multiple Schedules and Alarms
Recognising that sleep patterns may vary, the Apple Watch allows you to create multiple sleep schedules. For example, you can set different bedtimes and wake-up times for weekdays and weekends. You can also customise alarm sounds and choose whether you want your Apple Watch to wake you up with a silent, vibrating alarm.
Battery Life Considerations
To use the sleep tracking feature, your Apple Watch needs to be charged to at least 30% before you go to bed. If the battery level is lower, the watch will prompt you to charge it. Depending on the model of your watch, you may need to charge it during the day to ensure it lasts through the night.
Third-Party Sleep Apps
Before the introduction of native sleep tracking in WatchOS 7, Apple Watch users relied on third-party apps for sleep tracking. These apps offered varying levels of data and insights into sleep cycles. With the release of WatchOS 9, Apple has significantly enhanced its sleep tracking capabilities, providing a more comprehensive understanding of sleep patterns.
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How does it track sleep stages?
The Apple Watch uses signals from its accelerometer and heart rate sensor to detect when users are in REM, Core, or Deep sleep. The watch can estimate the time spent in each sleep stage and when the wearer woke up. The watch uses motion to detect sleep when worn to bed.
Third-party sleep-tracking apps are also available for the Apple Watch, and these use Photoplethysmography (PPG) to measure the user's heart rate. Dr. Dan Gartenberg, a sleep scientist and CEO of SleepSpace, explains that "from motion and heart rate data combined, we can develop algorithms that get close to what an electrode sensor would detect when it comes to our brainwaves."
Apple's VP of Technology, Kevin Lynch, has stated that the company's sleep studies have shown that the main thing when it comes to sleep is really about duration. He says that "movement of your arm is an input, but it's not a complete picture of what's going on inside your brain."
The Apple Watch's sleep tracking focuses on the quantity of time spent in bed, and it can tell when the wearer is awake in bed and using their phone.
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How accurate is it?
The Apple Watch uses an accelerometer, heart rate sensor, and blood oxygen sensor to approximate sleep stages. This is compared to the gold standard of measuring sleep stages in a lab, which is an EEG (electroencephalogram) device worn on the head to detect brain waves.
Apple claims that its machine learning models have been "trained and validated against the clinical gold standard, polysomnography, with one of the largest and most diverse populations ever studied for a wearable." The Apple Watch was found to be about 78% accurate in detecting REM sleep, with the most common error being the confusion between REM and core sleep (21% of the time). It rarely confused REM sleep with deep sleep or wakefulness (1% of the time).
Dr. Dan Gartenberg, a sleep scientist and the CEO of SleepSpace, states that devices like the Apple Watch "can estimate these states with about 80% accuracy." He also notes that the Apple Watch, along with similar devices like the Oura and Fitbit, use Photoplethysmography (PPG) to measure heart rate and motion data to develop algorithms that approximate the data an electrode sensor would detect in terms of brain waves.
Dr. Azizi Seixas, director of the Media and Innovation Lab at the University of Miami, expresses a note of caution, stating that "no two wearables estimate the same behavior (like sleep and step count) the same, and so users might be disappointed that fancy derived composite scores are only the company's 'black box' and generally unvalidated algorithm."
Dr. Valerie Cacho, a sleep medicine specialist, also highlights the potential for over-tracking, which may lead to ortho-insomnia, or insomnia caused by excessive sleep tracking. She emphasizes that "your subjective experience of how you feel in the morning can be a great measure of how good your sleep quality is." While technology can aid in this measurement, it should not override one's personal experience of sleep quality.
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How does it compare to other wearables?
The Apple Watch uses signals from the accelerometer and heart rate sensor to detect when users are in REM, Core, or Deep sleep. It can also track your breathing rate as you sleep, giving you greater insight into your overall health.
Compared to other wearables, the Apple Watch performs similarly to other devices in estimating two-state sleep (i.e. sleep or wake). It correctly identified 97% of sleep epochs, 26% of wake epochs, and 88% of all epochs, with a kappa value of 0.30, indicating a fair level of agreement. The Apple Watch overestimated sleep time by an average of 39.5 minutes and had proportional bias and heteroscedasticity.
Other wearables such as Oura and Fitbit also use Photoplethysmography (PPG) to measure heart rate and develop algorithms that approximate what an electrode sensor would detect in terms of brain waves. Dr. Dan Gartenberg, a sleep scientist and CEO of SleepSpace, states that these devices can estimate sleep states with about 80% accuracy.
A validation study of six wearable devices (Apple Watch S6, Garmin Forerunner 245 Music, Polar Vantage V, Oura Ring Generation 2, WHOOP 3.0, and Somfit) found that all devices detected more than 90% of sleep epochs but that Polar, Oura Gen 2, WHOOP 3.0, and Somfit outperformed Apple Watch and Garmin for detecting wake epochs. The study concluded that the Apple Watch and other devices included in the study could be used as an alternative to polysomnography for estimating two-state sleep but require improvement for assessing specific sleep stages.
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How do you set it up?
To set up sleep tracking on your Apple Watch, you'll need to make sure you have an iPhone with the latest version of iOS and an Apple Watch with the latest version of watchOS.
First, make sure your Apple Watch is charged to at least 30% before going to bed, with Sleep Tracking enabled. You can turn on Sleep Tracking by going to the Apple Watch app on your iPhone, hitting the Sleep tab, and toggling the slider for "Track Sleep with Apple Watch".
Next, you can set up your sleep schedule. Go to the Sleep app on your Apple Watch and follow the onscreen instructions. You can also do this through the Health app on your iPhone by tapping "Browse", then "Sleep", and then "Get Started" under "Set Up Sleep". Here, you can set a sleep goal (how many hours of sleep you want to get), your bedtime and wake-up times, and whether to turn on Sleep Focus, which limits distractions before you go to bed.
You can also create multiple sleep schedules, for example, one for weekdays and another for weekends. For each schedule, you can set the days, times, and an alarm sound.
To view your sleep data, open the Sleep app on your Apple Watch or the Health app on your iPhone. You can see the amount of sleep you got the previous night, the time spent in each sleep stage, and your sleep average over the past 14 days.
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Frequently asked questions
The Apple Watch uses signals from the accelerometer and heart rate sensor to detect when users are in REM, Core, or Deep sleep.
The Apple Watch has been found to correctly identify 97% of sleep epochs, 26% of wake epochs, and 88% of all sleep/wake epochs, indicating a fair level of agreement with clinical standards.
Apple trained its machine learning models against polysomnography, the clinical gold standard for sleep tracking, with one of the largest and most diverse populations ever studied for a wearable device.
Unlike other wearables such as Fitbit or Oura, which provide calculated sleep quality scores, Apple's sleep tech is more simplified, focusing on the duration of sleep, movement disturbances, and heart rate.
Ensure your Apple Watch has WatchOS 7 or later and your iPhone has iOS 14 or later. Charge your Apple Watch to at least 30% before bedtime and wear it to bed.
