Smartwatches are consumer gadgets, not medical tools say researchers from Leiden University, after a study found that wearables can’t tell the difference between stress and excitement.

Published in the Journal of Psychopathology and Clinical Science, the study – Associations between ecological momentary assessment and passive sensor data in a large student sample – set out to establish whether smartwatches could be used to replace self-reporting as a means of gathering data on factors impacting mental health. The aim was to establish whether it would be possible to build a warning system for depression in students using smartwatches.

Sleep, tiredness and stress were assessed, as they all impact the state of mental health.

Almost 800 students wore smart watches while answering EMA surveys on how they were feeling for three months.

The study found that while self-report and wearable measures of sleep-related variables showed robust associations, associations for tiredness were weaker and measures of stress didn’t overlap for most individuals.

For the majority of individuals in the sample, the self-report and physiological measures of stress showed 'very weak' to 'no' associations, suggesting that excitement might get flagged up as something negative.

“The data showed no meaningful correlation between self-reported stress and smartwatch readings – in a quarter of cases, the watch gave the opposite result,” said lead author and associate professor in the department of clinical psychology at Leiden University, Eiko Fried.

“The devices’ reliance on heart rate as a stress indicator proved misleading, as elevated heart rate can occur during both positive and negative experiences," he said.

"While body battery and sleep tracking were somewhat more accurate, these are consumer gadgets, not medical tools and should be treated with caution.”