Heart Rate Variability (HRV), a relatively new feature in wearables with optical heart rate monitors, is a good proxy for stress. Some years ago, IBM did a stud

Heart Rate Variability (HRV), a relatively new feature in wearables with optical heart rate monitors, is a good proxy for stress. Some years ago, IBM did a study that found that HRV in newborns served as an early warning system of illness. Athletes have long used this measure to determine when off days are needed.