Heart rate variability: what your watch is measuring
HRV is the tiny variation in timing between heartbeats. Wearables use it as a signal of recovery and stress. Here is how to read the number.
Kivorent News desk, , 2 min read
What to know
- HRV measures how much the time between heartbeats varies, in milliseconds.
- Your own trend over weeks matters more than any single reading or anyone else's number.
- Alcohol, poor sleep, illness and hard training commonly lower it for a day or more.
A heart beating 60 times a minute does not beat exactly once per second. One gap might be 0.95 seconds and the next 1.04. Heart rate variability, or HRV, is a measure of that variation.
Why the gaps vary
Your heartbeat is adjusted from moment to moment by two branches of the nervous system. One speeds things up when you need to act. The other slows things down when you are resting and recovering. When the resting branch is more active, the gaps between beats vary more. That is why a higher HRV, compared with your own normal, is generally read as a sign that your body is recovered.
How a wearable gets the number
Most watches and rings shine light into the skin and measure how much bounces back. Blood absorbs light, so each pulse shows up as a dip. The device times the gaps between pulses.
The measurement needs you to be still, which is why most devices take it during sleep. The figure usually reported is called RMSSD, a statistic that summarizes beat-to-beat differences in milliseconds. A chest strap that reads the heart's electrical signal is more precise. For tracking a nightly trend, an optical sensor on a still wrist or finger does a reasonable job.
What is a good number?
There is no single good number. HRV varies widely between healthy people and tends to decline with age. One healthy adult might average 25 milliseconds and another 90.
What you can use is your own baseline. Most apps build one over a few weeks and then show whether today is inside or outside your usual range.
What moves it
Things that commonly lower HRV for a night or longer:
- Alcohol, even a moderate amount
- Short or broken sleep
- An illness coming on
- A very hard workout or several in a row
- Mental stress
- A late, heavy meal
Things associated with a higher baseline over time include regular aerobic exercise and consistent sleep.
How to use it
- Look at the weekly trend. One low night means little. A week below your range is worth noticing.
- Compare like with like. Readings from different brands are calculated differently and are not interchangeable.
- Use it alongside how you feel. If the number says recovered and you feel wrecked, believe your body.
What it cannot do
HRV from a consumer wearable is a wellness signal. It does not diagnose a heart condition, and a low reading is not evidence of one. If you have symptoms such as chest pain, fainting or a racing heart at rest, that is a reason to see a clinician, whatever the watch says.
HRV is one of the inputs to the sleep and readiness scores most wearables show. We break those down in what a sleep score measures.
This article is general information about how technology works. It is not medical advice. Talk to a clinician about your own health.
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