The findings raise questions about whether wearables can reliably measure mental states, even as sleep tracking offered a modest bright spot
Consumer-grade smartwatches may be less reliable than many users think when it comes to tracking stress and fatigue, according to new research accepted for publication in the Journal of Psychopathology and Clinical Science.
The surprising findings land in a booming wearables market, where devices promise insights into sleep, energy levels and stress by measuring heart rate and other biometrics. However, when it comes to stress and tiredness, a group of researchers note that the numbers often don’t match what users actually experience.
For the study, a team led by Björn Siepe tracked nearly 800 university students over a three-month period, all of whom wore Garmin Vivosmart 4 devices while completing smartphone-based ecological momentary assessments (EMAs) four times a day. Each evening, they reflected on their best or worst experiences.
The data revealed that the overlap between smartwatch readings and self-reported states was minimal, particularly for stress. For example, stress readings on the devices seldom matched what participants reported.
One reason, co-author Eiko Fried explained, is that wrist-worn devices rely heavily on heart rate and its variability, which can rise for many reasons.
“Changes of heart rate alone tell us little about a person’s context: your heartbeat could be increased due to experiencing not only a negative, but also a positive emotion,” he said in a release. “For example, you could be afraid or excited, stressed or sexually aroused.”
As he told The Guardian, his wearable flagged him as stressed while he was working out and again when he was excitedly catching up with a friend at a wedding.
Fried said the findings also raise questions about the role of wearable data in understanding mental states and what it can or cannot reliably show. It’s an important aspect, as the work is part of WARN-D, a broader effort to build an early warning system for depression.
“Not only is this relevant for individuals who rely on information provided by smartwatches to guide their life habits,” he said. “It’s also important for clinical and social sciences, given the common belief among researchers that wearables can replace traditional self-reports to reduce research participant burden.”
While the wearables showed little consistency in stress or fatigue, sleep tracking proved more reliable, providing one bright spot. For about two-thirds of participants, better self-reported nights corresponded with roughly two extra hours of sleep recorded on the watch, though the devices measured duration rather than quality.
Ultimately, he suggested consumers take a cautious approach.
“Be careful and don’t live by your smartwatch – these are consumer devices, not medical devices,” he told The Guardian.
