HRV: The Complete Guide to Reading Your Nervous System
HRV is the beat-to-beat variation in the time between heartbeats, and more of it usually means a better-regulated nervous system. The number worth watching daily is RMSSD (your parasympathetic 'recovery' signal); SDNN is a broader capacity measure. HRV is deeply personal, so ignore population 'normal' charts and track your own baseline. Measure the same way every morning, watch a 7–14 day trend, never react to one day, and use it as one input among heart rate and the stand test, not a grade on your worth.
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