Heart rate variability (HRV) is not your heart rate. It's the variation in time between successive heartbeats — measured in milliseconds. A heart that beats at exactly 60 beats per minute with no variation between beats is actually a concerning sign. Healthy hearts beat with a natural fluctuation driven by the autonomic nervous system. Higher variability generally indicates better health, better recovery, and greater resilience to stress.
Why HRV Reflects Autonomic Balance
HRV is a window into the balance between your sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) nervous systems. The vagus nerve — the largest parasympathetic nerve — modulates heart rate beat by beat. When you're well-rested, recovered, and low-stress, vagal tone is high, and HRV is elevated. Under stress, illness, poor sleep, or overtraining, sympathetic dominance reduces vagal tone — and HRV falls.
What HRV Predicts
HRV has been studied as a predictor of cardiovascular disease, mortality, inflammatory disease, depression, and recovery from illness and surgery. Athletes use it to optimise training load — when HRV drops below individual baseline, the body signals it needs recovery rather than more stress. In clinical settings, reduced HRV is an independent risk factor for sudden cardiac death and adverse cardiac events.
HRV Metrics Explained
- RMSSD — the most commonly used HRV metric; reflects short-term vagal activity; best measured at rest, first thing in the morning
- SDNN — the standard deviation of all beat-to-beat intervals; reflects total autonomic activity over longer periods
- LF/HF ratio — the ratio of low-frequency to high-frequency power; higher ratios indicate sympathetic dominance
What Raises HRV?
- Consistent aerobic exercise — even walking regularly improves vagal tone
- Quality sleep — HRV is highest after deep restorative sleep
- Cold exposure — brief cold showers or cold water immersion activate the vagus nerve
- Controlled breathing — slow diaphragmatic breathing at 6 breaths/minute maximises HRV acutely
- Omega-3 fatty acids — consistently shown to raise HRV in supplementation trials
- Reduced alcohol — even moderate alcohol consumption suppresses HRV for 24+ hours
Measuring HRV
Consumer wearables (Apple Watch, Garmin, WHOOP, Oura Ring) all measure HRV and provide useful trending data. For clinical accuracy, a Holter monitor or validated chest-strap HRV measurement (e.g. with Polar H10 and a validated app) provides the most reliable data. Trend over time is more valuable than any single measurement.
