What is RMSSD and why does my wearable use it to measure HRV?

RMSSD (root mean square of successive differences) is the standard clinical measure of short-term heart rate variability, calculated as the square root of the mean of squared differences between successive R-to-R intervals on an ECG, it reflects primarily parasympathetic (vagal) nervous system activity and is the HRV metric most consistently associated with cardiovascular health outcomes in research (Task Force of the European Society of Cardiology, Eur Heart J, 1996).

RMSSD is preferred over SDNN (standard deviation of all NN intervals) for short-term measurement because it isolates the vagal component of autonomic regulation and is less sensitive to respiratory rate and other confounding variables. Wearable devices use RMSSD because it can be calculated from a short measurement window, the 5-minute overnight reading your Oura ring or Whoop takes during deep sleep is capturing the same metric cardiologists use in clinical autonomic assessments, though with lower precision than a lead-ECG recording.

Honesty Scale: Solid (1). RMSSD is the established research standard for short-term HRV assessment and its biological meaning is well-defined.

What to do: When comparing HRV readings, confirm your device reports RMSSD (not SDNN, not LnRMSSD, not a proprietary "readiness" score). SDNN includes sympathetic as well as parasympathetic components and runs higher than RMSSD for the same individual, the reference ranges are different.

For the full picture, read Your Whoop Is Worried. Here Is What It Actually Found.