What HRV actually measures (and what it doesn't)

Heart rate variability (HRV) is the variation in time between successive heartbeats. Even at a "steady" resting heart rate, the interval between beats fluctuates by tens of milliseconds. That variability reflects the moment-to-moment balance of sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) input from the autonomic nervous system.

What it represents#

Higher HRV at rest generally reflects greater parasympathetic activity (vagal influence on the sinoatrial node) and is associated with better cardiovascular fitness, lower stress, and good recovery from physical and emotional load.

Lower HRV at rest reflects either greater sympathetic dominance, reduced parasympathetic capacity, or both. It's associated with acute stress, illness onset, sleep deprivation, alcohol intake, overtraining, and as a stable trait, cardiovascular disease.

This is well-established in clinical cardiology. HRV depression after a myocardial infarction is a known predictor of poor prognosis.

The metrics#

The wearable industry uses one HRV metric more than any other: RMSSD (root mean square of successive differences). It reflects short-term, beat-to-beat changes and correlates strongly with parasympathetic activity.

Other metrics exist (SDNN, pNN50, frequency-domain metrics like HF and LF power) and tell different stories. For consumer use, RMSSD is the standard.

Some wearables report a "score" derived from RMSSD on a 1–100 scale; these scores are often log-transformed RMSSD or vendor-specific composites. They are not directly comparable across brands.

Why your number isn't your friend's number#

HRV is highly individual. A healthy 28-year-old athlete might have an RMSSD of 90; a healthy 50-year-old in average shape might have 35. Both can be normal. Genetics, age, sex, fitness, posture, time of day, recent meal, alcohol, caffeine, and breathing all affect the number.

Comparing your absolute HRV to anyone else's is mostly unhelpful. Comparing your HRV to your own baseline over weeks is where the signal is.

How HRV is captured#

The gold standard is an ECG. Chest straps (Polar H10, etc.) are nearly as accurate. Wrist-worn devices use photoplethysmography (light through the skin) which introduces noise; modern devices typically report HRV only during sleep when motion artefacts are low.

This is why most consumer HRV metrics are sleep-derived. Comparisons of awake HRV between wrist and chest strap show much larger discrepancies than sleep-derived measures.

What HRV is and isn't useful for#

Useful as:

• A trend signal: week-on-week direction, paired with subjective notes.
• An early warning: sudden drops often precede illness, particularly viral infections, by 1–2 days.
• A training-load proxy in athletes: overtraining typically shows up as a slow HRV decline.

Not useful as:

• A single-day verdict on your health.
• A measure of "stress" in any precise sense.
• A comparison metric between people or devices.

Like sleep tracking, HRV is best treated as a noisy diary, useful in aggregate, misleading in any single reading.