What is RMSSD?
RMSSD (root mean square of successive differences) is a time-domain measure of heart rate variability: it summarizes how much the interval between adjacent heartbeats changes from one beat to the next, in milliseconds.
It reflects vagally mediated changes in heart rate, which is why it is the standard metric for short recordings. Vagal tone itself cannot be measured directly, and a single RMSSD value is not a stress reading or a diagnosis. Values depend on age, breathing, posture, time of day and whether they come from an ECG or a wearable's pulse signal — so night-time wearable values and short daytime lab values are not interchangeable.
Most ring and strap wearables (Oura, Whoop, Garmin, Polar) report RMSSD; Apple Health has long recorded HRV as SDNN (see SDNN).
Read the science → RMSSD: what this HRV metric reflects, and what it doesn't — definition, measurement, evidence by strength and sources. Age tables: normal HRV by age.