What is RMSSD?
RMSSD (root mean square of successive differences) is the primary HRV metric reflecting short-term autonomic nervous system activity. It is the most widely used HRV measure in wearable fitness devices, sports science, and cardiac health monitoring.
What is SDNN?
SDNN (standard deviation of all normal-to-normal RR intervals) is a comprehensive HRV metric capturing overall autonomic variability over a recording period. It is used in clinical cardiology and long-term stress and health assessments.