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

ATLANTA– Your watch indicates that you slept three hours soundly. Should we believe it?

Millions of people rely on phone apps and wearable devices like rings, smartwatches, and sensors to monitor their sleep, but these trackers don’t necessarily measure sleep directly. Instead, they infer sleep states from signals such as heart rate and movement, raising questions about the reliability of the information and how seriously it should be taken.

The U.S. sleep tracking device market generated approximately $5 billion in 2023 and is expected to double its revenue by 2030, according to market research firm Grand View Research. As these devices continue to grow in popularity, experts say it’s important to understand what they can and can’t 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.

Whether it’s an Apple Watch, Fitbit, Oura Ring or one of countless other competitors, health and fitness trackers take largely the same basic approach by recording the user’s movements and heart rate while resting, according to Daniel Forger, a mathematics professor at the University of Michigan who studies the science behind sleep wear.

The algorithms used by major brands have become very accurate in determining when someone is sleeping, Forger said. The devices are also useful for estimating sleep stages, although a laboratory study would be more accurate, he said.

“If you really want to know for sure how long your non-REM sleep is versus REM sleep, that’s where lab studies really excel,” Forger said.

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

Branson says these patients are taking the wrong approach: The devices help highlight trends over time, but should not be considered a definitive measure of sleep health. Data from a single night should also not be considered significant.

“We would have believed them with or without the device and tried to figure out why they couldn’t sleep – and that’s what wearable devices don’t do,” she said.

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

Forger takes a more favorable view of the devices, which he says help keep the overlooked importance of sleep in mind. He even recommends them to people without significant sleep problems, saying they can offer insights that help users refine their routines and feel more alert during the day.

“Seeing if your body clock is in sync is a huge benefit, because even if you give yourself the right amount of time, if you sleep at the wrong hours, sleep won’t be as effective,” Forger said.

Kate Stoye, an Atlanta-area middle school teacher, bought an Oura Ring last summer after hearing positive things from friends who were using it as a fertility tracker: “It’s so accurate,” she said. Stoye found the ring to be just as useful for tracking his sleep. After noticing that the few nights she drank alcohol coincided with poorer sleep, she decided to quit alcohol.

“I don’t really see a reason to drink if I know it’s going to affect how I feel,” said Stoye, who always wears her device except when she’s playing tennis or needs to charge it.

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

“I always have trouble going to bed, and it’s often because I eat late at night,” Stoye said. “I know it about myself, and he knows it too.”

Mai Barreneche, who works in advertising in New York, wore her Oura ring constantly. She said it helped her develop good sleep habits and encouraged her to maintain a daily morning exercise routine. But as a metrics-driven person, she became “obsessed” enough about her nightly sleep scores that it began to cause her anxiety — a modern condition that researchers have dubbed “orthosomnia.”

“I remember going to bed thinking about what 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 maintained the good habits the device told her, but no longer wanted to go through the stress of monitoring her nightly scores.

Branson, of the Morehouse School of Medicine, said she has observed similar score-induced anxiety as a recurring problem in some patients, particularly those who set goals to achieve a certain amount of REM sleep or who share their nightly scores with friends using the same device. It’s not wise to compare sleep types and stages because individual needs vary by age, genetics and other factors, she said.

“These devices are supposed to help you,” Branson said. “And if it makes you anxious, worried or frustrated, it’s no use and you really should talk to a professional.”

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

“The body makes some really interesting and very important decisions that we’re not aware of to keep us healthy, active and alert at the right times of the day,” he said. “If you have an infection, that rhythm starts to disappear very quickly because the body kicks in to start fighting the infection. Those are the kinds of things we can detect.”

The technology could be particularly useful in low-resource communities, where wearable devices could help identify health problems more quickly and monitor them remotely without requiring access to doctors or specialty clinics, according to Forger.

“There’s a really important story that’s about to be revealed: about how understanding sleep rhythms and architecture is going to improve our lives overall,” he said.