HRV: What It Measures and What Moves It

Heart rate variability, HRV, is the small variation in the time between one heartbeat and the next. A resting heart rate of 60 does not mean a beat exactly every second. It means beats spaced at 0.95 seconds, then 1.05, then 0.98. The size of that wobble, measured in milliseconds, is the number your watch or ring reports each morning.

What the wobble means

The heart has its own pacemaker, and two branches of the nervous system adjust it. The sympathetic branch speeds it up for stress and effort. The parasympathetic branch, carried mainly by the vagus nerve, slows it down for rest and digestion. When the parasympathetic branch is active it nudges the pacemaker with every breath, speeding the heart slightly as you inhale and slowing it as you exhale. That nudging is most of what HRV measures.

So a higher HRV usually means the rest and recover branch has the upper hand. A lower HRV means the body is under load and the stress branch is holding the wheel. That load can be training, illness, alcohol, poor sleep or a hard week at work. The number does not say which.

Why one number is noise

HRV swings a lot from night to night. A 20% change between two mornings is common and means little on its own. It also differs widely between people. One healthy 45 year old sits at 30 milliseconds, another at 80, and both are fine. Comparing your number to a friend's, or to a chart, tells you almost nothing.

What is useful is your own baseline over a few weeks and how today sits against it. Most wearables show a rolling average and flag a reading that drops well below it. That is the right way to read it: the trend, not a single morning.

What moves it

• Alcohol. The most reliable lever most people have. Two or three drinks in the evening typically drop that night's HRV noticeably and raise resting heart rate, and the wearable shows it the next morning before you feel it.
• Illness. HRV can fall a day or two before symptoms start, as the immune system puts the body under load.
• Hard training. A heavy session or a race lowers it for a night or two. A steady slide across a training week is a sign to back off.
• Sleep. Short or broken sleep lowers it. Consistent bedtimes tend to go with a steadier, higher baseline.
• Age and fitness. HRV falls with age and rises with aerobic training, which is part of why the baseline is personal.

The practical use is as an early warning. A reading well below your baseline for two or three mornings in a row is worth taking seriously: ease the training, skip the drink, get to bed early, and watch it come back. If it does not, that is a reason to look harder at what is going on.

That's the kind of thing we look at alongside the labs.

This article is educational and is not medical advice.

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