Some health enthusiasts swear by smartwatches as a way to monitor stress levels, but a recent study calls into question that common usage. The study, published in the Journal of Psychopathology and Clinical Science, claims that such watches display a very limited ability to actually communicate what a person’s psychological state is. Sometimes, a watch may think the user is stressed when they’re really just excited about something, researchers say.

The report looked at nearly 800 students who wore a Garmin Vivosmart 4 smartwatch and measured their self-reported emotional states against the metrics collected by the wearables. According to the study, the self-reports of the watch-wearers and the analyses provided by the watches bore little resemblance to one another. It notes:

We investigated the concurrent overlap between self-report and wearable sensor data measuring stress, tiredness, and sleep. For the majority of individuals in our sample, we found that self-report and physiological measures of stress show very weak to no associations. These results raise several questions about differences between data sources and potential measurement issues.

Garmin advertises a stress-tracking capability for its smartwatches on its website. “Stress levels (0–100) are estimated by the Firstbeat Analytics engine, primarily using a combination of HR and HRV data. This data is recorded by the optical heart rate sensor on the back of your device.”

However, Garmin seems to admit that the quality and character of stress can be difficult to measure: “Public speaking and running up a flight of stairs can both send your heart racing, but the underlying reasons why are fundamentally different,” its website notes. The company suggests that wearing the watch more frequently can result in better measurements. “You can improve the quality of the insight gained by wearing your device as much as possible, especially while you sleep, because that is when your stress levels will typically be lowest,” the site states. “This helps create a better understanding of the full range of stress and relaxation states that you experience.”

In an interview with The Guardian, one of the study’s authors, Eiko Fried, said that the correlation between the self-reported stress scores that were collected as part of the study and the readings provided by the smartwatches was “basically zero.”

“This is no surprise to us given that the watch measures heart rate and heart rate doesn’t have that much to do with the emotion you’re experiencing – it also goes up for sexual arousal or joyful experiences,” he told the outlet. “The findings raise important questions about what wearable data can or can’t tell us about mental states,” he continued. “Be careful and don’t live by your smartwatch – these are consumer devices, not medical devices.”

The study’s topic has a diverse research history. A 2023 meta-analysis of studies about wearables and stress management found that “the effect of wearable-based approaches on alleviating or reducing stress” had “not been analyzed” and that most studies up until that point had “focused on presenting overviews of wearable devices.” Another study published by researchers at the Vrije Universiteit Amsterdam in 2023 found, much like the recent psychology study, that smartwatches frequently failed to distinguish between excitement and stress. Gizmodo reached out to Garmin for comment on the recent study and will update this story if it responds.

While the study claims Garmin’s wearable didn’t do much to measure stress, researchers found it seemed to provide decent metrics in other arenas. The report says that the watches were very good at measuring sleep, although it notes that “associations were weaker for tiredness.”