What is Heart Rate Variability?
Your heart doesn't beat like a metronome. Even at rest, the time between consecutive heartbeats varies slightly from beat to beat. This natural variation is called Heart Rate Variability (HRV).
Each interval between heartbeats (RR interval) is slightly different
Higher HRV generally indicates a healthier, more adaptable cardiovascular system. It reflects the balance between your sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) nervous systems.
Why Does HRV Matter?
- Stress indicator — lower HRV can indicate higher stress or fatigue
- Recovery tracking — HRV rises as your body recovers from exercise
- Autonomic health — reflects the flexibility of your nervous system
- Sleep quality — HRV patterns change during sleep stages
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Quick Reference
| Metric | Domain | Reflects |
|---|---|---|
| SDNN | Time | Overall variability |
| RMSSD | Time | Short-term (vagal) activity |
| pNN50 | Time | Parasympathetic tone |
| SD1 | Nonlinear | Short-term variability |
| SD2 | Nonlinear | Long-term variability |
| Baevsky SI | Time | Sympathetic stress (histogram shape) |
| HRV Score | Time | ln(RMSSD) scaled 0–100 |