Here’s the honest answer. Sleep trackers are good at telling when you’re asleep, decent at your total hours, and shaky on deep sleep. In one lab study, a popular watch came in 43 minutes short on deep sleep on average, while a ring matched the lab. Even trained sleep-lab scorers agree on deep sleep only about two-thirds of the time. So treat the deep sleep number as a rough trend, not a measurement.
I’m not a doctor, nurse or pharmacist, and this isn’t medical advice. A sleep tracker is not a diagnosis. If you snore loudly, wake up gasping or feel worn out every day, talk to your doctor no matter what the app says.
What a Sleep Lab Measures That Your Ring Can’t
To know how close your tracker gets, it helps to know what it’s being compared with. A sleep lab test is called polysomnography, and it’s a lot of wires.
Small sensors on your scalp read your brain waves. Four more sit around your eyes, two by each one, to catch eye movements. Others go on your chin and a leg for muscle activity, one goes on your chest for your heart, and belts around your chest and belly track your breathing. A clip on your fingertip reads your oxygen, and a camera and microphone record the night.
The brain waves are the key part. Light sleep, deep sleep and REM sleep are told apart mostly by brain-wave patterns, and your ring has no way to see those. It never touches your head — it sits on your finger or your wrist all night.
How a Ring or Watch Guesses Your Sleep Stages
So what is it doing all night? It’s watching your body and making an educated guess. The Oura ring’s sleep staging works from movement, temperature, heart rate and heart rate variability, the small changes in time between heartbeats. Watches and fitness bands use a similar mix of motion and heart signals.
Those signals do shift with each stage. Your heart tends to slow and steady in deep sleep, and you barely move. But a calm, still stretch of light sleep can look a lot like deep sleep from the outside, and that’s where the guesses go wrong.
Think of it like guessing what’s on TV in the next room from the light under the door. You’ll often be right about whether it’s on. You’ll be wrong more often about which show it is.
Asleep or Awake: The Part Trackers Get Right
Here’s the good news. In a 2024 study at a Boston hospital, 35 healthy adults aged 20 to 50 slept one night in the lab wearing an Oura Ring Gen3, a Fitbit Sense 2 and an Apple Watch Series 8. All three caught at least 95% of the time people were truly asleep, and all three came close to the lab on total sleep time.
Catching the awake time is harder. In a 2021 Navy study, 34 young adults spent three nights in a sleep lab wearing up to seven consumer trackers. The devices were great at spotting sleep but only fair to poor at spotting wake, and they did worse on the nights when sleep was broken up on purpose.
In plain terms, if you lie still in bed for 20 minutes waiting to drift off, your tracker may count some of that as sleep. Your total can come out a little rosier than your night really was, especially after a restless one.
READ: All You Need to Know About the Fitbit Sleep Score
Deep Sleep Is Where the Numbers Drift
Now the less good news. Sorting the night into light, deep and REM is a much harder job than asleep or awake, and the same Boston study shows it. The Fitbit came in 15 minutes short on deep sleep on average and 18 minutes long on light sleep. The Apple Watch was 43 minutes short on deep sleep and 45 minutes long on light sleep. The Oura ring wasn’t measurably different from the lab on any stage.
Those are averages across 35 people. On any one night, for any one person, the miss can be bigger or smaller, and an average of zero can hide a few misses in both directions.
Deep sleep is hard even for the lab. When more than 2,500 trained scorers graded the same sleep recordings, they agreed on the stage 82.6% of the time overall, but only 67.4% of the time for deep sleep. Telling the lightest deep sleep from regular light sleep was one of the places they disagreed most.
So when the display says you got 1 hour and 12 minutes of deep sleep, the fair reading is “somewhere around an hour, give or take.” That’s still useful (more useful than it sounds, honestly). It’s just not a lab result.
Ring, Watch or Band: Who Came Closest
In that 2024 study, the ring did best on stages. Oura picked the same stage as the lab 76.0 to 79.5% of the time, depending on the stage. The Fitbit landed between 61.7 and 78.0%, and the Apple Watch had the widest spread, from 50.5 to 86.1%.
Before you run out and buy a ring, read the fine print on that study. It tested one night, in healthy adults no older than 50, with no sleep disorders. One of the authors sits on Oura’s medical advisory board. And all three companies have changed their software since then, so the numbers on your wrist tonight may not match the ones in the paper.
Bottom line: any of the three is fine for watching your sleep over weeks. None of them is good enough to fret over a single night’s deep sleep.
READ: Smart Oura Ring Gen 3 for Sleep Tracking
What Throws the Reading Off
A few things make the guesses worse — and most of them are out of the tracker’s hands.
- A rough night. In the Navy study, the trackers did worse on nights with broken sleep, which are exactly the nights you’ll want to check.
- Lying still while awake. Reading quietly, or waiting out a 3 a.m. wake-up without moving, can be counted as sleep.
- A sleep disorder. The big lab studies mostly tested healthy younger adults. How well the stages hold up with sleep apnea or insomnia is far less studied.
- A software update. The stages come from the company’s own formula, and when it changes, your numbers can shift overnight even though your sleep didn’t.
That last one catches people off guard. If your deep sleep jumps or drops the week after an app update, look up what the app changed before you worry about yourself.
READ: Best Sleep Tracker for Seniors: 7 Picks
Don’t Let the Score Keep You Up
Sleep doctors have a name for worrying too much about tracker numbers. In 2017, a group of sleep researchers named it orthosomnia, after seeing patients come in for sleep problems they’d diagnosed from their own tracker data. Some of them ended up on a perfectionist chase for the ideal night, and the tracker felt more true to them than a lab test did.
Here’s what I’d do instead. I’d look at the weekly average, not last night. Put more weight on how you feel at 10 a.m. than on any number. And if checking the app first thing makes you anxious, it’s fine to take a week off from it.
Most adults need seven to nine hours of sleep, and that’s the number your tracker is best at. The total hours are worth watching. The deep sleep slice can stay in the background.
When the Numbers Mean Call Your Doctor
A tracker can’t tell you why you sleep badly. Some signs need a doctor no matter what your app says, and sleep apnea is the big one. Its common signs include loud snoring, gasping for air, breathing that stops and starts, and feeling sleepy or tired all day. Morning headaches and getting up often at night to use the bathroom can be signs too.
Some watches now have a sleep apnea alert. The FDA authorized Samsung’s Galaxy Watch feature in February 2024 to flag signs of moderate to severe sleep apnea in adults 22 and older. That’s a nudge, not a diagnosis. Only a proper sleep study can tell you whether you have it and how bad it is.
If you share a bed, your partner may be the best sleep tracker you have. If they tell you that you stop breathing at night, call someone that week. Don’t wait for the app to agree.
READ: Sleep Apnea Test at Home: A Detailed Explanation
Conclusion
Your sleep tracker is a good judge of when you fell asleep and roughly how long you slept. It’s a fair judge of your sleep stages, and a shaky one on deep sleep, where even lab experts disagree a third of the time. Use the weekly trend, check the fine print when the numbers jump, and let how you feel during the day count for more than the score.
And if anything about your nights worries you, the reading on your wrist is a reason to call your doctor, never a reason to skip the call. Sleep well!
