Heart rate variability: what the number means

There is no single good value, your watch and a clinical device will disagree, and the only comparison worth making is against yourself under the same conditions.

What is being measured

A healthy heart does not beat like a metronome. The interval between beats varies constantly, by small amounts, as the two branches of the autonomic nervous system adjust to breathing, posture and blood pressure. That variation is a sign of a responsive system, and it is quantified in two ways: in the time domain, from the statistics of the interval series, and in the frequency domain, from the rhythms present within it.

Counterintuitively, a heart beating with very little variation is usually the less healthy one. Loss of variability reflects loss of the flexibility that lets the system respond.

Why there is no good number

It falls with age, substantially and normally.
It varies enormously between individuals for reasons that have nothing to do with disease.
It moves with everything you did yesterday — sleep, alcohol, caffeine, a hard training session, an argument, the start of a cold.
It moves with medication — beta-blockers, anticholinergics, antidepressants and antiarrhythmics all change it, in different directions.
It depends on how it was measured — posture, breathing rate, recording length, time of day and the algorithm used.

A population reference range for such a measurement is close to useless for an individual. What is not useless is your own value, taken the same way, twice.

Why a wearable gives a different number

Different sensor, different sampling, different artifact rejection, different algorithm, and usually a different recording window — often overnight rather than a controlled resting period. Two methods can both be internally consistent and not comparable to each other. Compare within a method, never across.

How it is measured clinically

From an interval series derived from the pulse or the electrocardiogram, at rest under controlled conditions, and then during standardized challenges — deep timed breathing, standing, and a controlled exhale against resistance. The challenges are where most of the clinical information is, because they ask a specific branch of the system to respond and measure whether it does. Cardiac autonomic reflex tests.

What to do with a low value

Repeat it under controlled conditions before concluding anything. If it is genuinely and repeatedly low, the useful next questions are about medication, sleep, glycemic control, deconditioning and any of the conditions that cause autonomic neuropathy. It is a signal to investigate, not a diagnosis.

Autonomic symptoms and the full test description.