What sleep trackers can and cannot measure

A sleep tracker overestimates the time you spend sleeping, sometimes by as much as an hour

I got into a conversation with a guy at a party on Saturday, and he said, “Those sleep trackers, they’re not all that reliable, right?” I realized the answer couldn’t be summed up in a single sentence.

That’s why the recent study by researchers at the University of Salzburg is so timely. Eight popular sleep trackers were compared to polysomnography, the medical gold standard for sleep research.

Multiple sleep trackers

For the study, participants slept five nights at home while wearing multiple wearables simultaneously, including the Oura Ring 3, Apple Watch Series 9, Fitbit Charge 6, WHOOP 4.0, Garmin Venu 3, and Circul+.

At the same time, their sleep was measured using polysomnography, which records brain activity, eye movements, and muscle activity. The researchers also examined how wearables handle irregular sleep patterns, such as short sleep, nighttime interruptions, and spending an extra-long night in bed.

The accuracy of the devices varied significantly. The most accurate were sensors that directly measure heart rate variability via a chest or arm sensor, achieving 84% accuracy in recognizing sleep stages. However, consumer wearables scored significantly lower, as low as 56%.

Overly optimistic sleep

Notably, almost all wearables overestimate how much a person sleeps—sometimes by up to an hour per night. This is because many trackers use movement as a key indicator. If someone lies still while awake—such as when worrying—the algorithm often interprets this as sleep.

De foutmarge nam sterk toe als deelnemers korter sliepen, vaker wakker werden of langer wakker lagen. Dat is geen fijn signaal voor mensen met slaapproblemen, die veelal eerder geneigd zijn om hun slaap te meten, omdat ze controle zoeken.

Sleep Wearables Aren’t Worthless

This doesn’t mean sleep wearables are useless, however. They can still be genuinely useful for tracking sleep rhythms, identifying patterns over time, and monitoring behavioral changes. But the exact number of minutes of deep sleep or REM sleep should be taken with a grain of salt.

Ultimately, the most important measure of a good night’s sleep is you. How rested do you feel during the day? Can you concentrate well? Do you have the energy to get through the day, or do you feel sluggish and irritable? No wearable can fully capture that in a score or graph.