What HRV actually tells you, and what it doesn't

Heart rate variability is the number people quote at each other. "My HRV tanked." "My HRV is 38, is that bad?" It has become a kind of shorthand for how well you are coping, which is unfortunate, because it is one of the least transferable numbers your wrist produces.

Here is a plain account of what it is, and where it stops being useful.

What is actually being measured

Your heart does not beat like a metronome. Between one beat and the next there are small differences, measured in milliseconds. HRV is the spread of those differences over a window of time, usually while you sleep.

Those differences are not noise. They come from the autonomic nervous system, the part of you that runs breathing, digestion and heart rate without being asked. It has two branches, roughly a "rest and recover" branch and a "get ready" branch, and the balance between them shows up in the spacing of your heartbeats. More variability generally means the resting branch has more of the floor.

That is the whole of the physiology most people need. Everything after it is interpretation.

Why your number and your partner's number cannot be compared

Two healthy adults of the same age can have overnight HRV values that differ by a factor of three or four. Not because one is fitter, but because HRV varies enormously between individuals for reasons that have nothing to do with how anyone's week is going: genetics, resting heart rate, age, where the sensor sits, and the algorithm the manufacturer uses.

The last one matters more than people expect. Devices differ in what they measure and how they average it, so the same night on two wrists can produce two very different numbers. Comparing your figure with a friend's is comparing two different measurements that happen to share a name.

What is comparable is your number against your own number, last week. That is the only comparison worth making, and it is the one every decent app is quietly doing on your behalf.

What a drop usually means

A meaningfully lower HRV than your own recent average tends to follow things you could probably have named yourself:

• A short night, or a fragmented one.
• Alcohol, particularly in the few hours before sleep.
• A hard training session the day before.
• Being ill, or about to be.
• Sustained psychological stress, the kind that does not stop when you close the laptop.

Notice what that list has in common. Every item is a load on the body, and none of them are specific. A single low reading tells you that something was demanding. It does not tell you which thing, and it never tells you why.

What it definitely does not mean

It is not a mood reading. There is no "sad" signature in heart rate variability and anyone selling one is selling you a correlation dressed as a diagnosis.

It is not a diagnosis of anything else either. A low night is not evidence of illness, and a high night is not evidence of health. Isolated readings are weak; trends across weeks are where the signal lives.

And it is not a score to beat. This is the failure mode we see most often: someone starts checking HRV each morning, a low number puts them in a bad mood, the bad mood costs them sleep, and the number goes down again. The measurement starts causing the thing it was supposed to observe.

How to use it without letting it run you

Three rules cover almost everything.

Look at the week, not the morning. A single night is mostly noise. Seven of them is a pattern.

Ask what changed, do not accept a verdict. The number is a prompt for a question you can answer with your own memory. You were on a plane, the room was too warm, the argument went on until one in the morning.

Let it inform the day, not define it. "Maybe today is not the day for the hardest session of the week" is a good use. "I am broken" is not, and it is not what the data says.

Why Pulse never shows the number to your partner

This is the reasoning behind a decision people ask us about constantly. Pulse reads your resting heart rate, HRV, sleep and movement on your own device, and your partner never sees any of them.

Partly that is privacy. Mostly it is that the number would not help them.

If your partner sees "HRV 31", they have three options, and two of them are bad. They can worry, having no context for the figure. They can perform concern, which is worse. Or they can ask you what it means, which puts you in the position of explaining your own physiology on a day you already have nothing spare.

What actually helps them is the thing the number implies, in language a person can act on: a calm evening might mean a lot today. That sentence carries the useful part of a low reading with none of the false precision. It leaves room for the context only you have, and it suggests something to do.

Your body's measurements are yours. What your partner needs is not access to them. It is a decent guess about how to show up.

If you want the longer version of that argument, we wrote it up in why your partner will never see your heart rate.