India, Aug. 13 -- Smartwatches promise a quick read on your day: heart rate, sleep, stress, and a nudge to breathe when things spike. A new peer reviewed study suggests one of those signals may be far less reliable than users think. When it comes to stress, consumer wearables can confuse excitement for strain and flag you as overworked when you are simply having fun.

Researchers tracked 800 young adults wearing Garmin Vivosmart 4 bands for three months. They compared the devices' stress, fatigue, and sleep scores with how participants said they felt in the moment. The headline result was blunt. On stress, there was "basically zero" correlation with self reported feelings, according to lead author Eiko Fried of Leiden University. He noted...