Sleep trackers can be misleading, experts say
They also caution consumers against looking to devices to cure insomnia
Shawn McCall, 48, a personal trainer in Waterford, Michigan, started tracking his sleep almost six years ago. McCall said his Oura Ring — a sleek titanium device that he wears on his ring finger — had revealed how the choices he makes during his waking hours affect his sleep at night.
“It serves as a constant reminder that if I do certain things, like have too much to drink or eat a large meal before bed, I know my heart rate is going to be higher that night and I’ll definitely have less deep sleep,” he said. “It helps keep me accountable.”
The popularity of consumer sleep-tracking technology has grown rapidly in recent years, and that growth is projected to continue.
Led by the Oura, Fitbit and Apple Watch and other wearable devices, the market includes phonebased apps and “nearables,” which are placed on or beside a person’s bed.
While the capability and sophistication of sleep trackers vary, they can record such things as heart rate, movement, body temperature and blood oxygen levels. Using these data, trackers claim to offer sleep insights, such as estimates of nightly deep sleep or a total “sleep score” that reflects overall sleep quality.
Experts who study sleep trackers say that while there are some benefits to knowing this data, it can be presented in misleading ways, and they caution that sleep trackers aren’t a cure for insomnia or other sleep disorders.
How they work
Many wearable devices collect data using something called photoplethysmography, or PPG.
“There’s this little light on the back of the device that shines into the blood vessels, and it uses the amount of light that is reflected back to estimate things like heart rate and heart rate variability,” said Dr. Cathy Goldstein, a clinical professor of neurology at the University of Michigan who studies consumer sleep monitors.
She added that most wearables, and many nearables, contain accelerometers that can measure motion.
Sleep trackers that are placed under a mattress (or, in the case of some “smart beds,” inside the mattress) often rely on ballistocardiography, a technology Goldstein said is capable of detecting subtle movements caused by the heart’s pumping action — movements that correlate with sleep and its stages.
Researchers have found that the latest sleep trackers are generally adept at detecting the basics: when a person is asleep or awake. However, Goldstein said trackers can be less accurate when collecting data from people with obesity or heart rhythm disorders, as well as from those who have darker skin tones, because skin pigment can interfere with how light is reflected to the device.
But even if the data collection is perfect, experts say, many of these technologies overreach when they attempt to translate findings into consumer-friendly takeaways.
“They’re presenting information with a granularity that they’re not yet capable of,” Goldstein said.
For example, while many trackers offer data on a user’s sleep stages, such as REM sleep and deep sleep, these stages are defined by shifting patterns of brain activity — something most devices can’t directly measure.
“Inferring sleep and the sleep stages from peripheral phenomena like pulse rate or respiration has some inherent limitations, especially if the person is not healthy,” said Mathias Baumert, an associate professor of biomedical engineering at the University of Adelaide in Australia who specializes in health technology.
It’s also unclear how people could benefit from this information. “We don’t diagnose sleep disorders based on REM sleep or deep sleep,” said Kelly Baron, a clinical psychologist and director of the Behavioral Sleep Medicine program at the University of Utah. She said that even people who sleep well have differing patterns of REM or deep sleep as a result of age, sex, medication use and other variables.
“I have patients who come in saying they’re worried because their device is telling them they’re not getting enough deep sleep, but I couldn’t even tell you how much deep sleep is optimal,” Baron said.
Experts are especially critical of sleep trackers’ attempts to aggregate a person’s nightly data into an overall sleep grade or score. In a 2022 paper, Baumert and his coauthors pointed out that the algorithms companies use to determine these scores are often proprietary and not scientifically vetted.
“A simple metric is attractive from a consumer perspective,” Baumert said. “But it is difficult to understand what is measured and what those scores mean in terms of health outcomes and disease.”
Goldstein put it more bluntly: “Ugh, these sleep scores or readiness scores are the worst. I tell my patients to ignore those.”
How to use them
One of the advantages of sleep trackers is their ability to capture and record long-term data in a person’s natural sleeping environment. “No matter how sensitive the equipment, a night spent in a sleep lab is not representative of a hundred nights of sleep at home,” Baron said.
The potential to spot meaningful long-term patterns in one’s sleep — the way McCall noticed how alcohol and late-night meals messed with his slumber — is “extremely exciting, both for sleep scientists and for people who have these devices,” Baron said.
Goldstein said the data from these devices could also reinforce the benefits of sleep hygiene. For example, a user could see how going to bed and getting up at the same time each day positively affects their metrics.
On the other hand, people who are anxious about their sleep may want to think twice before using a tracker. Research on realworld users has found that these devices can stress people out or increase their focus on sleep, which can be counterproductive.
“If you’re not sleeping well, having this device that tells you how poorly you’re sleeping might make things worse,” Goldstein said.
Finally, it’s important to remember that there’s a lot about sleep that remains a mystery. “There is still so much we need to learn about the role of sleep, and how patterns of sleep and sleep disruption affect health,” Baron said.
“I think the current devices can be fun for people and provide some interesting information. But sleep can’t be boiled down into a set of numbers or scores.”