Heart-rate variability, without the jargon
The most useful signal in consumer wearables is also the most badly explained. A plain-language guide.
Tafadzwa Moyo · 22 May 2026 · 6 min read
Your heart does not beat like a metronome. The gap between beats varies constantly, by milliseconds, and that variation is a window onto how your nervous system is coping.
More variation is generally better
When you are rested, the branch of your nervous system that handles rest and digestion has the upper hand, and the beat-to-beat gaps vary more. When you are stressed, ill, dehydrated, short on sleep or deep in a training block, that variation compresses.
Why you should ignore other people's numbers
Genetics, age and measurement method move heart-rate variability enormously. A perfectly healthy person can sit at 30 milliseconds while their equally healthy training partner sits at 120. Comparing them is meaningless. Comparing you to your own average from last fortnight is not.
| Score | 48 of 100 |
|---|---|
| Zone | Moderate readiness |
| Meaning | How prepared your body is today, from sleep, resting heart rate and heart-rate variability. |
How Kairo uses it
We sample through your deepest, stillest sleep, throw away segments corrupted by movement, and feed the clean result into Readiness alongside your resting heart rate and how much of your sleep need you met. You never have to look at a millisecond figure — but it is there if you want it.
One caution worth repeating: this is a wellness signal, not a diagnosis. A run of unusual readings is a reason to rest and, if it persists, to speak to a doctor — not a verdict from your wrist.
