Heart rate variability (HRV) is the variation in the time between two consecutive heartbeats over a specific period. Doctors may use HRV as an indicator for certain aspects of a person’s health, ...

Millions of people now start their day with a number—a "readiness" score, a "body battery"" level or a measure of "strain""—delivered by the wearable device on their wrist or finger. But how much ...