Your smartwatch is great for logging workouts, mapping routes, and storing your playlist. But if you use it to track sleep, know this: Research on wrist-worn sleep trackers, including a 2019 meta-analysis of 22 different studies, found that they detect the correct sleep stage only about half of the time.
Why Its Important to Avoid Running Through Injury slow-wave sleep—the deepest and most restorative sleep stage that can help runners recover from heavy training—you need a new strategy.
Sleep stages correspond to brain waves, something that even highly sophisticated devices can’t measure, explains Conor Heneghan, Ph.D., lead research scientist at Fitbit. To Run Faster, Sleep Slower heart rate, then process that information into an algorithm to interpret what’s happening in the brain. Sometimes the algorithm correctly identifies the corresponding sleep stage, but often it doesn’t. Individual differences in movement and heart rate also make sleep stats difficult to measure on a device, says clinical psychologist and sleep specialist Michael Breus, Ph.D. And smartwatch sleep data may be even less reliable for insomniacs or those with a movement disorder such as We may earn commission from links on this page, but we only recommend products we back, says Heneghan.
The good news: Your tracker is pretty good at measuring your overall time spent asleep—and if you get enough hours between the sheets, you’re probably getting enough deep slow-wave sleep.
“For athletes, I look at total sleep time, since that has a reasonable degree of accuracy, then use that to transition the conversation to sleep habits,” says endurance coach Jason Koop, head coach for CTS in Colorado Springs.
Heneghan notes that looking at the data pattern over time is the best way to use wearable trackers to measure deep sleep. “If your [tracker indicates that] you’re getting one hour of deep sleep, and you give up caffeine and find you’re getting 80 minutes, that’s a significant result.”
Still, technology is evolving quickly, and new offerings are in the pipeline. Fitbit Premium’s new “restoration” score is more about quality of sleep than quantity: it uses a combination of physiological metrics that correlate with sleep quality like total sleep, and nighttime heart rate, to tell users how restful their sleep was.
Then again, you may want to try a low-tech approach to sleep tracking: pencil and paper. Certified sleep health educator and sleep coach Martin Reed, M.Ed., recommends using an analog sleep diary to track when you go to bed, when you get up, and the way you feel after sleeping. “How you feel, how well you’re performing, and overall sleep time are probably the best measures of sleep quality.”
