HRV explained: why your number only matters compared with your own normal

Heart rate variability, or HRV, measures the small differences in time between one heartbeat and the next. Those differences are shaped partly by your nervous system: when you're rested and relaxed the gaps tend to vary more; when you're under strain they tend to even out.

Why your number isn't comparable

HRV varies hugely between people, and between devices. Watches measure it in different ways and at different times, often overnight. Two people can both be in great shape with very different numbers. That's why a single HRV figure, or a "good" or "bad" label from a chart, doesn't mean much on its own.

What's worth watching

What's useful is how your HRV moves compared with your own usual range. Things that often go with a lower reading than usual:

poor or short sleep
hard training without enough recovery
alcohol
stress, illness or travel

And things that often go with a higher reading than usual: good sleep, well-judged training and calmer weeks. A single low night is normal. A run of them is worth noticing, and maybe easing off a little.

A slow drift is easy to miss

The changes that matter most are often the slowest. A month of HRV drifting a little lower each week rarely triggers a single dramatic alert, but put next to how you've been feeling and how much you've been training, it can tell a clear story: you've been asking a lot of your body and not giving much back. Looking at the trend over weeks, rather than each morning's reading, is what makes that visible.

Don't chase the number

HRV is best treated as a gentle signal, not a score to beat. Trying to push it up day by day tends to add stress rather than reduce it. Watch the trend over weeks, pay attention to how you actually feel, and if the two disagree, trust how you feel.

Tempest describes your own data; it doesn't diagnose or treat anything. If you're worried about your health, speak to your GP or call NHS 111. In an emergency, call 999.