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

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. 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. "If you really want to know definitively how much non-REM sleep you're having versus REM sleep, that's where the in-lab studies really excel," Forger said. 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. This article was provided by The Associated Press.