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How the Apple Watch tracks sleep -- and why

Apple Watch chief Kevin Lynch explains the company's sleep approach in a conversation with CNET.

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- 07:12
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Sleep tracking on WatchOS 7 is overdue, and it's starting with bedtime guidance.

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It's been almost five years since I first had a sleep study done at home. I put wires all over my face, wore a pulse oximeter on my finger and confirmed I had sleep apnea. I've used a CPAP machine ever since, and I've been interested in understanding how well I sleep and how to improve my rest. I've tried tons of watches, bed sensors and apps. Of all the smartwatches I've tried, the Apple Watch was conspicuously the only one that seemed to lack its own onboard sleep tracking.

While sleep tracking was possible using other Apple Watch apps, Apple never entered the space formally, despite purchasing sleep-tech company Beddit in 2017. That's changed with WatchOS 7, unveiled last week at Apple Worldwide Developers Conference. It adds sleep tracking as an onboard feature, but you might be surprised at what sleep tracking on Apple's watch does and doesn't do. 

Unlike other wearables such as the Fitbit or Oura, which measure how much time you spend in the various sleep phases and even give calculated sleep quality scores, Apple's sleep tech is more simplified. It just tracks duration of sleep, movement disturbances and heart rate. The content of your sleep isn't analyzed much at all. Instead, Apple's placed a big focus on the time you go to bed and what you do while you wind down.

Apple's VP of Technology, Kevin Lynch, who has worked on the Apple Watch from the beginning, says that sleep has been part of the plan since the watch debuted five years ago. The company's been researching sleep tech for years, running private sleep studies and using EEGs to measure sleep against what a device like the Apple Watch can record. Lynch has a unique perspective on why other features, like estimated sleep scores, aren't being presented for now. In a conversation with CNET during WWDC, he shared some thoughts on why Apple's sleep tech is what it is.

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Apple's Wind Down feature on the iOS 14 is part of the Apple Watch's sleep experience, too.

Sarah Tew/CNET

It's about the wind down, not a sleep score

I tried Apple's Bedtime feature on iOS way back in 2016, which was a surprisingly helpful tool to remind me to stick to a good bedtime routine. The iPhone feature lets you set your bedtime and morning alarm times and then tracks whether you use your phone during the downtime. Even then, I saw it as something that would eventually make sense on the Apple Watch. 

Wind Down is an expansion of that feature, and it also connects to helpful apps that could be part of that routine. My 11-year-old son already does things to wind down each night, including meditation, and I should learn from him. That's what Apple's feature aims to help with. 

Apple sees the wind down and the bedtime routine as an important part of sleep quality. 

"Many sleep apps show information about REM cycles and other data like that," Lynch says. "And we've looked a lot into that."

But Apple's research has found something else is more important: the amount of time you sleep. "Even in our studies, we had people wear EEGs on their heads, so we got insight into the electrical activity of their brain, in addition to what we're able to sense on the wrist with Apple Watch," Lynch says. "And we've learned a lot about how the main thing here is really about duration."

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Lynch says that Apple decided not to show more complicated estimated sleep data because the company's sleep studies didn't show results that felt useful: "Movement of your arm is an input, but it's not a complete picture of what's going on inside your brain," he says.

Third-party apps can show deeper estimated sleep analysis, but Apple believes "that it can be overwhelming and stressful to look at that data," Lynch says.

"You can't really coach yourself to have more or less REM stages," he says. "We felt like that wasn't the best way Apple could add value here on sleep. We focused on the transition to the bed, which we think is way more actionable, and will result in people getting a better night's sleep, which then has secondary effects of perhaps your REM stages sorting themselves."

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Duration and schedule, but deeper estimated analysis isn't available yet.

Apple

Positive reinforcement only

Sleep patterns can be tracked in Apple's Health app, but there's a conscious move away from pinging people to fix bad habits or alerting them to nights with suboptimal sleep. Instead, notifications and reinforcements are all positive, similar to what Apple has done previously in the Health and Activity apps. If a sleep goal is met or beaten, there's positive feedback. If the goal isn't met, the app will aim not to flag it. 

"There could be anxiety that people have about going to sleep, and that anxiety itself can actually cause more problems in terms of going to sleep," Lynch says. "Many people are already well aware that they haven't been getting enough sleep, and so we're not adding to that, but we are positively acknowledging when you have achieved your goals."

Apple's not planning to use your sleep data

Companies like Fitbit have used anonymized user data for years to study demographic sleep patterns and improve observations on sleep, but Apple doesn't plan to do that. Instead, Apple has been building its machine learning models from data collected through internal studies.

"And that takes a long time. So we've been working on this for a while," Lynch says. "We treat the data that's being collected on a user's device with a high level of sensitivity around privacy. …  Apple is not seeing your sleep data."

Like other sensitive information collected by Apple devices, stats about your sleep stay on your device. You control where it's shared, like with third-party apps connected to HealthKit, and Apple doesn't collect the information.

No public sleep studies, yet

Apple has public health studies for areas like heart health, hearing and women's health, which are enabled through separate opt-in research apps, it has no public sleep studies yet. Instead, "has been doing our sleep studies internally with thousands of people," Lynch says. Apple could eventually open a public sleep study, if the company intends to study deeper links to conditions like sleep apnea. 

Fitbit -- now acquired by Google -- has enabled blood oxygen measurement on its recent trackers with the aim to provide deeper sleep analysis or sleep apnea detection, although no FDA-cleared results have taken place yet. The connection between fitness trackers and accurate sleep analysis has been fuzzy. I've worn an Oura ring lately and have been using its sleep scores as a semi-helpful snapshot of my restfulness the night before. Does that score always line up with how I feel? No. I wear my CPAP machine, and it's not aware of what my sleep trackers are recording. Or vice versa.

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Current Apple Watches still need charging nearly every day, and sleep tracking needs a 30% charge for a night's recording.

Angela Lang/CNET

Battery life works for now, but longer battery would make sense

Existing Apple Watches will need at least 30% battery life on a charge before tracking sleep at night. If the battery level's lower, the watch will request a charge-up. The Apple Watch 5's battery lasts about 18 hours, and you can charge the battery to 80% in about 1.5 hours and 100% in 2.5 hours. Apple's sleep tracking dims the display and reduces background activity, showing a basic watch face and the date or alarm if a button is pressed. 

If you're used to charging the Apple Watch every night, wearing the watch to bed may seem like a disruption. "That does introduce new charging behavior for people where before they would just charge overnight," Lynch acknowledges, leaving wearers to decide whether to top off in the morning, charge a bit more at night, or just skip sleep tracking.

It would make a lot of sense to just find a way to improve the Apple Watch battery life -- maybe by expanding the battery capacity in the next models or removing Force Touch, which senses pressure to do things like change the watch face. Apple's new app guidelines encourage developers to move away from Force Touch, and include these features in the UI or through a long press on WatchOS 7. Apple removed pressure-sensitive 3D Touch from the iPhone 11 Pro and gained considerable battery life in return. Could the same happen for the Apple Watch this fall and help sleep tracking be less awkward, too?

WatchOS 7 doesn't formally arrive until the next Apple Watch launches later this year, so there's still a lot we won't know. But Apple's offering a public beta for WatchOS 7 for the first time this summer, so you'll be able to try sleep tracking out for yourself.

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.