HRV Percentile by Age.

Heart rate variability is the beat-to-beat variation in time between heartbeats. High variability reflects strong parasympathetic ("rest and digest") tone and a flexible autonomic nervous system; chronically low variability tracks stress load, poor recovery, and — at the population level — higher cardiovascular and all-cause risk. RMSSD is the metric most wearables report and the most portable between devices.

Two caveats keep this honest. First, HRV falls predictably with age, which is why a fair comparison is age-banded. Second, it is intensely individual and sensitive to measurement conditions (night vs spot-check, posture, device) — so your own multi-week baseline and its trend tell you far more than any single absolute number or percentile.