Smartwatches typically measure stress using a metric called Heart Rate Variability (HRV). HRV is the variation in time intervals between consecutive heartbeats. It is a measure of the beat-to-beat changes in heart rate and reflects the body's response to various stimuli, including stress.
Higher HRV is generally associated with a more relaxed and less stressed state, while lower HRV can indicate increased stress or fatigue. Smartwatches with stress monitoring capabilities use the HRV data along with other inputs like heart rate, physical activity, and sleep patterns to provide users with insights into their stress levels and overall well-being. The measurements are typically displayed as a stress score or presented on a stress scale, allowing users to understand their stress patterns and make lifestyle adjustments as needed.