What Is Heart Rate Variability and Why Does Your Fitness Watch Measure It?

Did you know you can configure Google to filter out junk? Follow these steps to improve your search results, including adding my work on Lifehacker as a preferred source .

In addition to measuring heart rate and other metrics like step count, almost all fitness trackers and smartwatches measure heart rate variability (HRV). This metric underlies recovery and stress assessments and can usually be found as a separate metric in the “Vital Signs” section of your chosen app. Let’s explore what this metric actually measures and how to interpret it.

HRV is heart rate variability.

HRV stands for heart rate variability, but it’s not what you might think. HRV isn’t concerned with changes in your heart rate throughout the day or during exercise. (Your heart beats faster when you exercise intensely, which is what heart rate zones measure. That’s a completely different purpose.)

Heart rate variability determines the rhythm of your heartbeats. If the time between heartbeats is always the same (at the same heart rate), you have low heart rate variability. But if some heartbeats are faster and others are slower, you have high heart rate variability.

It may seem surprising, but it’s actually important to have at least some heart rate variability. A high heart rate variability is considered better than a low one. If your heart rate variability was higher than usual last night, your fitness app will tell you you’re well-recovered, while if your heart rate variability was low, it will tell you you’re stressed or recovering poorly.

However, of course, heart rate can be extremely variable. In a sense, this is what constitutes arrhythmia. When I talk about heart rate variability (HRV), and when fitness apps report your HRV data, they’re taking into account the subtle differences between heartbeats in a generally normal (more or less) healthy person.

These differences are measured in milliseconds, and you wouldn’t notice them unless you had a watch that records data while you sleep. If you ever notice anything unusual in your heart rhythm, consult a doctor (or, if necessary, seek emergency medical attention).

What does high heart rate variability mean?

A high heart rate variability (meaning high for you ) usually means you’re relatively relaxed and not too stressed. Fitness apps will boost your readiness and recovery if your heart rate variability (HRV) during sleep is high.

Heart rate variability is thought to be caused by the interaction of two different systems in the body: the sympathetic (“fight or flight”) and the parasympathetic (“rest and digest”). It’s normal for the brain and body to maintain a balance between these needs, so this is considered normal functioning.

What does low heart rate variability mean?

Low heart rate variability (HRV) typically occurs when the body is under stress—recovering from an intense workout, fighting a cold, or experiencing severe stress. If your HRV, measured while you sleep, is low, you’ll get lower readiness or recovery indicators. I feel like I’m in reserve when I’m sick or going through difficult times, both physical and psychological. (The day after a race or weightlifting competition is always a low HRV day for me.)

What HRV value is considered “good”?

I’ll give you some numbers in a minute, but the most important thing to know is that heart rate variability is most useful when compared to your normal range . Your heart rate variability may be higher than mine but still be low for you, or vice versa.

So, please don’t ask whether an HRV of 50 is “good.” If it’s higher than usual for you ? Then it’s good. If it’s lower than usual? Then it may indicate your body is experiencing stress. If your typical HRV values ​​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 100; these numbers are just fun facts about the general population. They don’t apply to you. Research generally shows that people who are physically fit have higher HRV, but also that HRV declines with age. The average man’s HRV is slightly higher than the average woman’s, but the range of “normal” values ​​for both sexes is almost exactly the same.

Another important point to note: when examining heart rate variability (HRV) data, you’ll find that different studies and different devices measure HRV differently. For example, this meta-analysis found that the average HRV was 42, with a range of 19 to 75, when measured as rMSSD (mean squared serial differences).

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

• An analysis of Whoop users’ heart rate variability (HRV) data shows that men have an average heart rate variability of 40, while women have an average heart rate variability of 37. They have graphs showing how this changes with age, highlighting the differences for each group.

• An analysis of the heart rate variability of Oura users shows that the average value for all users is 41, and they also plot it by age and gender.

As I discovered when comparing HRV readings on several devices , devices don’t always show identical HRV values, but they usually all show similar trends. If you’re sick, all devices will show low HRV. If you’re having a good day, all devices will show high HRV. So, pay more attention to the trend than to the details.

More…