Journal / Science
HRV is the number everyone quotes and nobody explains
Heart rate variability is the most useful recovery signal you have, as long as you read it against yourself.
Open the bitshealth app in the morning and the first thing behind your readiness score is HRV. It is two digits, measured in milliseconds, and it goes up on good days and down on bad ones. That much is right. Almost everything people say beyond that needs a caveat.
What it measures
Your heart is slowed by the vagus nerve, part of the system that runs rest and digestion. When that system is active, your heart speeds up slightly as you breathe in and slows as you breathe out, and the gap between beats wobbles by tens of milliseconds. When you are stressed, ill or under-recovered, the wobble shrinks.
We measure that wobble overnight, in the last hours of sleep when your body is stillest, and summarise it with a statistic called RMSSD.
Why your number is not comparable to anyone else's
Age, genetics and fitness all move the absolute value. A 25-year-old runner might sit at 90 ms and a healthy 55-year-old at 30 ms. Both can be in perfect shape. That is why the app always shows your number against your own 30-day baseline, never against other people.
How to use it
- Look at the seven-day trend, not the nightly value.
- Two or more nights 15 percent below your baseline is a signal to ease off, check for illness, or look at sleep and alcohol.
- Do not try to raise it directly. Regular sleep, aerobic training and less alcohol lift it over months.