Every watch now reports heart rate variability, and almost nobody knows what to do with it. The number goes down, you feel vaguely guilty, you train anyway. Or it goes down, you skip a session, and it turns out you had simply been drinking the night before.
HRV is genuinely useful, but only if you read it the way it works: as a trend against your own baseline, not as a daily score.
One low reading means nothing
Daily HRV is noisy. Values bounce in and out of your normal range for reasons that have nothing to do with training. What matters is the baseline underneath, and whether it is holding or falling.
- Daily reading
- 7-day baseline
- Your normal range
Illustrative data. Absolute HRV values are highly individual and are not comparable between people: a good number is one that is normal for you, which is why the band matters more than the axis.
What it actually measures
Your heart does not beat like a metronome. The gap between beats varies slightly, and heart rate variability is a measure of that variation.
That variation is largely controlled by your autonomic nervous system, which has two branches: the sympathetic, which drives stress responses, and the parasympathetic, which handles rest and recovery. Higher variability generally reflects greater parasympathetic activity, meaning a body in a recovered, adaptable state. Lower variability generally reflects a body under stress, whether from training, illness, poor sleep, alcohol, or work.
Note "stress", not "training stress". HRV cannot tell the difference between a hard interval session and a bad week at work. That is a limitation and, used properly, a feature: both genuinely affect what you should ask of yourself today.
Why the daily number is nearly meaningless
Day-to-day HRV is noisy. A single reading is affected by when you measured, how you slept, what you drank, whether you are getting a cold, and ordinary biological variation.
This is why comparing your number to someone else's is pointless, and why chasing a higher number day to day leads nowhere. Absolute HRV values vary enormously between individuals for reasons that have nothing to do with fitness. A value that would be alarming for one person is perfectly normal for another.
What is meaningful is your own pattern:
- Where your baseline sits, usually a rolling average over about a week.
- How wide your normal range is, so you know what counts as genuinely outside it.
- Whether several consecutive readings fall below that range, which is the signal worth acting on.
One low day is not a signal
A single reading below your range is noise most of the time. Three or four in a row, or a baseline that is trending down across a week or more while training load is high, is the pattern that actually means something.
Measuring it consistently
Consistency matters more than precision. Whatever you do, do it the same way every time.
Overnight measurement during sleep, which most modern watches and straps do automatically, is the most practical option and the least susceptible to being disturbed by how you got out of bed.
Morning measurement, taken immediately on waking, lying still, before caffeine or looking at your phone, is the classic protocol and works well if you are disciplined about it.
Either is fine. Mixing them is not, because they produce different numbers. Give any new measurement method three to four weeks before trusting its baseline, since you need a baseline before deviations from it mean anything.
Using it to decide what to train
The useful question is not "is my HRV good" but "does today's reading change what I should do".
Inside your normal range: train as planned. This is most days, and the correct response is to stop thinking about it.
Below range, one day, feeling fine: train as planned. Consider whether an obvious explanation applies, alcohol, a late night, a hard session yesterday, and get on with it.
Below range for several days, especially with high training load: this is worth acting on. Reduce intensity rather than volume: keep the easy running, move or soften the hard session. Suppressed HRV plus a hard interval session is how a heavy week becomes a lost fortnight.
Below range with other symptoms, elevated resting heart rate, sore throat, unusual fatigue: rest, and stop consulting the app.
Above range, well above baseline: usually just a good day. It is not an instruction to add a session.
The other half of this is intensity control, which is where your heart rate zones come in. When recovery data says back off, the practical action is usually holding your easy runs genuinely easy rather than cancelling them.
What HRV cannot tell you
Honesty about the limits, because this metric attracts more certainty than it deserves.
It cannot tell you why it dropped. It cannot distinguish productive training fatigue, which is the point of a hard block, from the unproductive kind. It responds to things unrelated to training. And a stable HRV does not prove you are not overreaching, since it is one signal among several, alongside resting heart rate, sleep, mood, and how the training itself is actually going.
Treat it as one input with real value and no authority.
The part that is hard to do yourself
The interpretation is where this falls down for most people. It requires holding several things at once: your baseline, your recent training load, where you are in a training block, and what today's session was supposed to be. Then deciding whether the sensible move is proceed, soften, or move it.
That is a daily judgement call, and it is exactly what Vaast automates. It reads your recovery data alongside the training you have actually completed, and adjusts the session in front of you rather than leaving you to interpret a number on a watch face.