HRV, explained

Heart-rate variability is the most quoted and least understood number in consumer fitness. It is genuinely useful and almost always used wrongly.

Meaningless

• Your number vs a friend’s
• One morning in isolation
• Across different devices

Meaningful

• Your trend vs your own baseline
• A sustained drop with feeling flat
• Measured the same way each time

What it measures

The variation in time between consecutive heartbeats. A healthy, rested heart does not beat like a metronome — the gaps flex slightly with breathing and autonomic activity.

More variability generally reflects a nervous system that is not under load. Less variability tends to show up under physical stress, illness, poor sleep, alcohol and psychological stress.

Why your number means nothing next to a friend’s

Absolute HRV varies enormously between individuals — age, genetics, fitness and measurement method all move it. Someone with 40 ms is not less healthy than someone with 90 ms.

It also depends on how it was measured: overnight averages, a morning reading and a chest-strap capture are not the same measurement, and different devices compute it differently.

The only meaningful comparison is your own number against your own baseline.

How to actually use it

Look at the multi-day trend, not one morning. Single readings are noisy.

A sustained drop alongside feeling flat is worth acting on — lighter training, earlier night, more food.

A drop while you feel fine is information to file, not a reason to cancel a session.

Measure consistently: the same way, at the same time, or let a wearable take it overnight while you sleep.

What it cannot tell you

It is not a diagnosis, not a measure of fitness, and not a readiness score on its own — readiness models combine it with sleep, load and other signals precisely because HRV alone is too noisy to steer by.

Questions