This morning, I woke up to the alarm on my smartwatch. The first thing it displayed was a morning report, which detailed my sleep last night. According to my Garmin, I got six hours and 37 minutes of sleep, one hour and 25 minutes of it spent in deep sleep, four hours in light sleep, one hour in REM (rapid eye movement, the sleep phase where most dreams occur), and a few minutes awake. It deemed my sleep quality as “good” and gave me a sleep score: 80 out of 100.
Sleep data like this is readily available on most fitness trackers and smartwatches, and companies like Oura and Whoop make devices dedicated to tracking rest and recovery. They claim to track sleep duration, sleep stages, and quality. But how accurate — or helpful — are these devices and the data they give? Can a small sensor on my wrist really tell me all about my REM cycles — and make qualitative claims about my sleep?
What do sleep trackers track?
Most devices use a sensor to track heart rate and an accelerometer to track movement, and some can track blood-oxygen levels. From those metrics, they attempt to paint a picture of your sleep: how much time you spent asleep, tossing and turning, and/or awake. Sometimes these devices will give you an overall sleep “score” or tell you how much more sleep you need the following night based on previous nights.
Is sleep-tracking data reliable?
These devices cast a wide net, so to speak, and pick up all sorts of heart-rate data and movement throughout the night. But interpreting the data isn’t as simple as your watch might make it seem. “They’re not particularly reliable,” says Dr. Dianne Augelli, sleep-medicine expert at Weill Cornell Medicine and NewYork–Presbyterian. “They can under- or overestimate based on movement, and so if your bed partner moves, if you have a pet, or if you have some underlying other issue, it can generate a lot of false data there,” she says.
I’m often not sure what to make of my heart-rate chart from a previous night’s sleep. When looking at my sleep chart from my Whoop, for example, I noticed spikes in my heart rate throughout the night. “We do have some heart-rate variability during sleep, and there can be some spikes there, but it’s hard to — in isolation — tell you exactly what’s happening there,” says Augelli. Those spikes in heart rate could be concerning, or they could be completely normal, she explains. You need more context to fully understand what’s going on at night — and see if it warrants more evaluation.
To get the most accurate sleep data, you’ll need more than a smartwatch.
Augelli’s biggest issue with sleep trackers is that they give information about sleep stages (like what my Garmin told me this morning), from just heart rate and movement, when in fact you’ll need more than just a watch to paint a fuller picture of your sleep. Dr. Mandana Mahmoudi, clinical director of sleep medicine at NYU Langone Health, agrees. “They absolutely do not have the ability to tell you when you’re in deep sleep, when you’re in superficial sleep, and how many times you’re actually waking up,” she says.
The gold standard for sleep research is an in-lab sleep study, but in most cases, that won’t be necessary for the average person. “When you do a sleep study, we have to put EEGs on you to look at your brain-wave activity,” Augelli says. “We have to look at muscle activity in order to be able to [understand] sleep stage. So all of this information that they’re giving you about deep sleep and all of this is generally not at all reliable, and I don’t recommend that people put very much faith into that at all.”
Don’t focus too much on the data you get.
The sleep doctors I spoke to emphasized trusting how you feel, rather than what your tracker tells you. “A good number of my patients walk into my clinic and they tell me what their tracker has to say about their sleep, and they forget to ask themselves, ‘How do I actually feel in the mornings when I wake up? Do I feel rested?’” Mahmoudi says.
Another thing to consider is that the way consumer wearables come up with the data they present to you is usually opaque and possibly not grounded in good science. These companies use a lot of proprietary information to analyze your data, Augelli explains. “Is it peer reviewed? We don’t know how well it works. They’re not usually testing these on a wide swath of individuals that have other sleep disorders or individuals on medications,” she says, noting that there are other factors that are often unaccounted for.
And focusing too much on the data might cause more harm than good. Augelli says she sees some patients who are very obsessed with their sleep-tracking data. “That causes more insomnia and more difficulty with sleep as they’re looking and they’re poring over this data,” she says.
But some sleep-tracking features might work for you.
While my watch might not be a reliable source for understanding how I’m sleeping, it can be a useful tool for establishing better habits. For example, I know that I don’t feel as good the morning after a night when I’ve been drinking, or when I’ve doomscrolled on my phone for an hour before bed. I’ve found, in general, that tracking my sleep and logging these activities (whether on my tracking device or on my phone or with a pen and paper) is helpful to me when I can frame my sleep in that context. I don’t need a heart monitor and an accelerometer to tell me that drinking affects my sleep — just the act of tracking or logging it helps. While Augelli and Mahmoudi don’t recommend their patients use sleep trackers, they do think they could be used for pattern recognition and making positive actionable changes. Besides logging caffeine or alcohol consumption, you might rely on a sleep tracker to make sure that you go to bed on time. “If the data triggers change toward good behavior, then I think it’s good if it can be motivational, like how a fitness tracker motivates people to do their 10,000 steps,” Mahmoudi says. “For patterns, I think they’re great, but not for specific percentages of certain stages of sleep.”
Still, it will be a while before Augelli recommends those devices to her patients. And it might require some more advanced technology than what’s currently on the market. “The kind of data that we get right now from consumer wearables — it’s not there yet.”
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