Wearables and fitness trackers like the Fitbit Air and Apple Watch may have a worse grasp of your sleep than you assume.

According to a study published in Sleep Medicine , wearable devices do not provide good sleep summaries for a significant number of people.

It suggests that while the results produced by wearables are OK for good sleepers, they’re actually quite poor for those who have trouble sleeping.

“Commercial wearable sleep trackers demonstrate poor to moderate agreement with validated subjective sleep quality measures, with significant population-specific variation,” reads the study’s conclusion.

The study abstract suggests that while the total reported amount of sleep is generally estimated with a decent level of accuracy, there are significant issues with other specifics in the data.

Fitbit Versus Traditional Methods

Results also show generally “poor agreement” with other sleep measuring techniques for those with insomnia or “clinical populations,” those who have a diagnosable sleep issue — insomniac patients’ wearable readings were only 39.4% agreement with other sleep-assessing techniques, for example. The review suggests wearables are not necessarily a good judge of sleep quality, beyond the basics of sleep duration.

There are some important factors to note to use as a counter, though. This study is actually a review of five existing studies, collating and analyzing their data to see what can be drawn from it.

The studies also use wearables that are quite ancient in tech terms. They include the Apple Watch models as early as the Watch Series 7 from 2021, the Fitbit Flex from 2013 and Fitbit Charge 2 from 2016. Manufacturers would have a pretty good argument that this meta study’s findings are largely irrelevant at this point, given the significant development in both sensor hardware in recent years, and the algorithms that process their data — since those devices were considered current.

One of the changes Apple just made in the Apple Watch Series 12 is emblematic of these developments. It has more of a focus on heart rate variability (HRV), which is a key indicator of bodily stress.

HRV is the average variance in the time between heart beats. Higher HRV is considered better, not as you might assume the low HRV you’d seen in a truly metronomic heart beat.

Back in the era of the Fitbix Flex, HRV was generally not in consideration as the heart rate hardware needs a good degree of fidelity to have the first clue as to what’s really going on with heart rate variability.

The study is certainly an indicator that wearables had poor sleep tracking. It’s less clear whether trackers like the Fitbit Air and Apple Watch Series 12 still have poor sleep tracking today.