What HRV Is, and Why Your Fitness Watch Measures It

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Besides measuring heart rate and other metrics like steps, pretty much every fitness tracker and smartwatch measures heart rate variability, abbreviated as HRV. This measurement is the basis for recovery scores and stress readings, and you can usually find it as a standalone measurement in the “vitals” section of your chosen app. So let’s break down what this number actually measures, and how to interpret yours. 

HRV is heart rate variability

HRV stands for heart rate variability, but that doesn’t mean what you might think. HRV is not related to your heart rate changing throughout the day or during exercise. (Your heart beats faster when you’re exercising hard, which is what heart rate zones measure. Totally different purpose.) 

Heart rate variability refers to how rhythmically your heart beats. If the time from one beat to the next is always the same (at a given heart rate), you have low HRV. But if some heartbeats come more closely together, and some are more spaced out, you have high HRV. 

This may be surprising, but you actually want your heart rate to show some variation. High HRV is considered to be better than low HRV. Your fitness app will say you’re well recovered if your HRV was higher than usual last night, and it will say you’re stressed or not recovering well if your HRV was low. 

That said, it’s of course possible to have a heart rate that is too variable. In a sense, that’s what an arrhythmia is. When I’m talking about HRV here—and when fitness apps report your HRV—they’re considering the teeny tiny differences from beat to beat in an otherwise normal(ish) healthy(ish) person. 

These differences are measured in milliseconds, and you wouldn’t notice them if you didn’t have a watch sampling data while you sleep. If you ever notice anything wildly strange about the rhythm of your heart, ask your doctor (or seek emergency medical help, if appropriate). 

What does a high HRV mean? 

High heart rate variability (that is, high for you) tends to mean that you’re relatively relaxed and not too stressed. Fitness apps will bump up your readiness and recovery scores when your HRV measured during sleep is high. 

The variation in beats is thought to come from the push and pull of two different body systems, the sympathetic (“fight or flight”) and parasympathetic (“rest and digest”) nervous systems. It’s normal for your brain and body to be balancing these needs, so this is considered a good, normal functioning. 

What does a low HRV mean? 

Low HRV tends to occur when your body is working on something—recovering from hard exercise, fighting off a cold, experiencing lots of stress. If your HRV measured during sleep is low, you’ll get lower readiness or recovery scores. I find my HRV tanks when I’m sick or when I’ve been through a rough time either physically or mentally. (The day after a race or a weightlifting competition is always a low HRV day for me.) 

What is a “good” HRV number? 

I’ll give you some numbers in a minute, but the most important thing to know is that HRV is most useful when you compare readings to what is normal for you. Your HRV can be higher than mine, but still low for you, or vice versa. 

So please don’t go around asking whether an HRV of 50 is “good.” Is it higher than usual for you? Then it’s good. Is it lower than usual for you? Then maybe it’s indicating that your body is under stress. If your typical HRV readings range from 35 to 50 (for example), then 50 is high for you. 

It doesn’t matter whether the average person your age has a HRV of 20 or of 100; those numbers are just fun facts about the population in general. They don’t apply to you. Studies tend to show that fitter people have higher HRV, but also that HRV declines as we age. The average man’s HRV is slightly higher than the average woman’s, but the full range of what’s “normal” for each gender almost fully overlaps.

There’s one more important caveat: when you look up HRV data, you’ll find that different studies and different devices each measure HRV differently. For example, this meta-analysis found the average HRV was 42, with a range of 19 to 75, when measured as rMSSD (root mean square of successive differences). 

So—and remember this is just for fun, here are some graphs from two wearable companies that publish their data and they both use rMSSD:

• Whoop’s breakdown of HRV data from their users finds men have an average of 40 and women 37. They have graphs breaking down how this changes with age, and showing the variation for each group. 

• Oura’s breakdown of HRV from their users finds an average of 41 across all users, and they also graph it by age and gender. 

As I’ve found when comparing multiple devices on HRV, devices don’t always report the same HRV numbers, but usually you can follow similar trends on all of them. If you’re sick, they’ll all report low HRV. If you have a good day, they’ll all report high HRV. So pay more attention to the trend than the specifics.

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