Heart Rate Variability (HRV)
The beat-to-beat variation in time between heartbeats — a non-invasive proxy for autonomic nervous system balance.
What it is
A healthy heart does not tick like a metronome. Each beat falls a few milliseconds earlier or later than the last, governed by the constant push-pull of sympathetic (accelerator) and parasympathetic (brake) input. HRV measures the size of that variability over a window — usually overnight or first thing in the morning. Most wearables report it as RMSSD in milliseconds.
Why it matters
Higher HRV generally means better-tuned recovery, fitness, and stress resilience. Lower HRV — sustained, not for a single night — flags overtraining, illness, poor sleep, or chronic stress. Absolute values vary widely between people, so the useful signal is your own trend. It is one of the few objective biomarkers you can move with breathwork, exercise dose, alcohol, and sleep within a week.