Heart rate variability: what it is and what it isn’t

HRV has become the headline number on every wearable. It does say something real — but far less precisely than the apps suggest.

A healthy heart does not beat like a metronome. Even at complete rest the interval between beats varies slightly from one to the next, and heart rate variability — HRV — is simply a measure of how much. Counter-intuitively, more variation is generally the better sign.

Why variation is good

Your heart rate is under continuous adjustment from two branches of the autonomic nervous system. The sympathetic branch speeds things up; the parasympathetic branch, working mainly through the vagus nerve, slows things down. The two are constantly negotiating, and the beat-to-beat variation is the visible residue of that negotiation.

When the parasympathetic side has good influence — you are rested, recovered, not under acute pressure — the adjustments are frequent and fine-grained, and variability is higher. Under stress, illness, or after hard training, sympathetic activity dominates, the negotiation stops, and the rhythm becomes more uniform. A very steady resting heartbeat is not a sign of a well-tuned engine; it is closer to the opposite.

What it can reasonably tell you

HRV is genuinely responsive to things worth noticing. It tends to drop with poor sleep, alcohol, acute illness, and heavy training load, and to recover as those resolve. Used as a personal trend, tracked at a consistent time — typically overnight or first thing in the morning — it can be a useful early signal that you are more depleted than you feel.

Where the apps overreach

Three things are worth knowing before you take a daily HRV score seriously.

• Measurement method matters. Chest straps read electrical activity directly. Wrist and finger devices infer beat timing optically, through blood flow. The optical approach is more convenient and less precise, and precision is exactly what a variability measure depends on.
• Context dominates. Body position, breathing rate, time of day, caffeine, and how well you slept all move HRV substantially. Comparing this morning to yesterday morning only works if the conditions were similar.
• A "readiness score" is an interpretation, not a measurement. The underlying number may be real; the recommendation built on top of it is a proprietary guess about what it means for you.

A sensible way to use it

If you already have a device, treat HRV as one input among several, and give more weight to the obvious ones: how you slept, how you feel, whether you are getting ill. Watch the multi-day direction rather than any single reading. And do not let a low number talk you out of a walk — light activity almost always helps recovery rather than hindering it.

If you do not have a device, you are not missing much. Sleep duration and consistency, resting heart rate, and simply how you feel on stairs carry most of the same information for free.

What HRV is not

HRV is not a diagnostic test and it is not a screening tool for heart disease. An irregular pulse, palpitations, chest tightness, breathlessness on mild effort or fainting are all reasons to see a doctor directly — they are assessed with an ECG and a clinical examination, not with a wearable's daily score.