In a nutshell: Millions of people are using smartwatches and other wearable devices every day to keep their stress levels in check. Almost all of them are wrong, researchers say, because there appears to be no correlation between how the body feels and digitally recorded data.
A recently published study says there is "basically zero" correlation between data coming from wearable sensors and self-reported stress levels. However, smartwatches and wearable gadgets aren't entirely useless if the user is well aware of the device's limitations.
The Dutch study tracked stress, fatigue, and sleep data for three months on 800 young adults, using Garmin Vivosmart 4 activity trackers. The researchers asked the study's participants to report how stressed, fatigued, or sleepy they felt four times per day.
After cross-referencing the data, the researchers discovered that little correlation existed between the data and the participants' personal experience. None of the 800 subjects got a "stress score" on their tracker that matched the level of stress they were feeling. Furthermore, a quarter of the participants were feeling stressed or relaxed while their smartwatches were measuring exactly the opposite.
Co-author Eiko Fried noted that the study's results aren't surprising. The Garmin watch they used measures heart rate, and this particular indicator has nothing to do with the actual emotions felt by a person. For example, heart rate usually goes up just as much during sexual arousal as when someone is angry.
"The findings raise important questions about what wearable data can or can't tell us about mental states," said Fried, warning, "[You should] be careful and don't live by your smartwatch – these are consumer devices, not medical devices."
The study also looked at the reliability of Garmin's "body battery" feature, which should measure physical fatigue. The relationship between that data and actual personal experience was stronger than mental stress, but still too weak to be relevant. Garmin doesn't explain how the body battery score works, but the researchers suspect it's a combination of pulse measurements and physical activity levels.
While smartwatches are mostly useless as stress meters, they could work better for sleep measurements. The tested devices helped measure sleep duration, even though they could tell very little about how rested a person would feel once out of bed.
This kind of sleep-related data could help developers create a "warning system" for depression states, the researchers posited. The watch could alert users when it predicts an episode is about to begin, so they could act beforehand or get the most appropriate treatment.
"Wearable data can offer valuable insights into people's emotions and experiences," said University of Manchester researcher Margarita Panayiotou. "But it's crucial to understand its potential and limitations."