Sleep-tracking devices have limits. Experts want users to know what they are

ATLANTA — Your watch says you had three hours of deep sleep. Should you believe it?

Millions of people rely on phone apps and wearable devices like rings, smartwatches and sensors to monitor how well they're sleeping, but these trackers don't necessarily measure sleep directly. Instead, they infer states of slumber from signals like heart rate and movement, raising questions about how reliable the information is and how seriously it should be taken.

The U.S. sleep-tracking devices market generated about $5 billion in 2023 and is expected to double in revenue by 2030, according to market research firm Grand View Research. As the devices continue to gain popularity, experts say it is important to understand what they can and cannot tell you, and how their data should be used.

Here's a look at the technology — and why one expert thinks its full potential has yet to be realized.

What your sleep tracker actually measures

Whether it's an Apple Watch, a Fitbit, an Oura Ring or one of innumerable other competitors, health and fitness trackers largely take the same basic approach by recording the wearer's movements and heart rate while at rest, according to Daniel Forger, a University of Michigan math professor who researches the science behind sleep wearables.

The algorithms used by major brands have become highly accurate for determining when someone is asleep, Forger said. The devices are also somewhat helpful for estimating sleep stages, though an in-lab study would be more precise, he said.

The sleep numbers that matter most

Dr. Chantale Branson, a neurologist and professor at the Morehouse School of Medicine, said she frequently has patients showing up with sleep scores from fitness trackers in hand, sometimes fixated on granular details such as how much REM sleep they got on a certain night.

Branson says those patients are taking the wrong approach: the devices help highlight trends over time but should not be viewed as a definitive measure of one's sleep health. Nor should any single night's data be seen as significant.

''We would have believed them with or without the device and worked on trying to figure out why they can't sleep — and that is what the wearables do not do,'' she said.

Branson said she thinks people who check their sleep statistics every morning would be better served by spending their efforts on ''sleep hygiene'' such as creating a relaxing bedtime routine, avoiding screens before bed and making sure their sleep environment is comfortable. She advises those concerned about their sleep to consult a clinician before spending money on a wearable.

''Seeing if your biological clock is in sync is a huge benefit because even if you're giving yourself the right amount of time, if you're sleeping at the wrong times, the sleep won't be as efficient,'' Forger said.

How sleep data can drive better habits

Kate Stoye, an Atlanta-area middle school teacher, bought an Oura Ring last summer, having heard positive things from friends who used it as a fertility tracker: ''It's so accurate,'' she said. Stoye found the ring to be just as helpful with tracking her sleep. After noticing that the few nights she drank alcohol coincided with poorer sleep quality, she decided to give up alcohol.

''I don't see much reason to drink if I know that it's going to affect how I feel,'' said Stoye, who always wears her device except when she is playing tennis or needs to charge it.

Another trend she says she detected in the ring's data: the importance of not eating too late if she wants to get good rest.

When sleep tracking becomes a problem

Mai Barreneche, who works in advertising in New York City, used to wear her Oura Ring constantly. She said it helped her develop good sleep habits and encouraged her to maintain a daily morning exercise regimen. But as a metric-driven person, she became "obsessed" enough with her nightly sleep scores that it began to cause her anxiety — a modern condition that researchers have dubbed ''orthosomnia.''

''I remember I would go to bed thinking about the score I was going to get in the morning,'' Barreneche said.

Barreneche decided not to wear her ring on a beach vacation a few years ago, and when she returned home, she never put it back on. She said she has maintained the good habits the device pointed her toward, but no longer wants the stress of monitoring her nightly scores.

Branson, of the Morehouse School of Medicine, said she's observed similar score-induced anxiety as a recurring issue for some patients, particularly those who set goals to achieve a certain amount of REM sleep or who shared their nightly scores with friends using the same device. Comparing sleep types and stages is ill-advised since individual needs vary by age, genetics and other factors, she said.

The future of wearables

Forger thinks the promise of wearables has been underestimated, with emerging research suggesting the devices could one day be designed to help detect infections before symptoms appear and to flag sleep pattern changes that may signal the onset of depression or an increased risk of relapse.

''The body is making these really interesting and really important decisions that we're not aware of to keep us healthy and active and alert at the right times of day,'' he said. ''If you have an infection, that rhythm very quickly starts to disappear because the body goes into overdrive to start fighting the infection. Those are the kind of things we can pick up.''

The technology could be particularly useful in low-resource communities, where wearables could help health issues to be identified more quickly and monitored remotely without requiring access to doctors or specialized clinics, according to Forger.