Heart rate variability (often referred to as HRV) is the measure of the variation in time between each heartbeat. These variations are so small that unless you’re hooked up to a specialized device you won’t be able to detect them for yourself, but they’re nonetheless incredibly important to your overall health and wellbeing.
HRV is a metric that is recorded by many of the best fitness trackers, including the Apple Watch, Garmin Fenix 7 Sapphire Solar and several Fitbit bands. It shows you how well your body is able to respond to signals from the parasympathetic and sympathetic nervous systems (the latter controlling the body’s fight-flight response and the former acting as brake) and adapt to the change in environment.
The results give you a good indicator of fitness, fatigue and stress, with a higher HRV typically considered positive, but making sense of a wearable’s heart rate variability readings requires a bit more context than your resting heart rate or — the athlete’s favorite — VO2 Max.
Read on to find out more about what HRV actually is, and what it may suggest about your health and fitness.
What is heart rate variability?
HRV is a measurement of the difference in the intervals between your heart beats over time. If your heart were to beat, robot-like, at 60bpm there would be a 1000ms interval between your heart beats. But that’s not how our hearts work. If one beat lands 1024ms after another, then the next is 1080ms later, that’s a variability of 56ms.
While there are several mathematical ways of calculating HRV, they all deal in how bad a metronome our hearts would make — it’s an average of these timing disparities within a time window. Heart rate variability also is a more precise measurement than your heart rate because of the tolerances involved.
For example, a fitness tracker may give you an HRV reading of, say, 68ms. This is equivalent to an average variability equivalent to a little over 4bpm during the period of testing. But if your heart rate reader is only accurate to within a handful of bpm of the real result anyway, the data becomes largely meaningless. (For context, a lot of the best budget fitness trackers don't even measure HRV, as it's so difficult to accurately track it.)
What does HRV mean for your health?
Why should we care about HRV in the first place? A stack of studies have been undertaken over the years that link it to several different aspects of health. A meta-analysis published in Psychiatry Investigation concluded that “HRV is impacted by stress” and that it can be used in the “objective assessment of psychological health and stress.” A low HRV score is an indicator of stress, then.
A large data set study by Fitbit’s research team that was published in The Lancet found HRV is impacted by exercise, making it a potential metric to judge how well you are managing your workout routine. HRV goes down directly after exercise, but after recuperating a regime that progresses your fitness should see a general upward trend in HRV results.
One 2018 study published in Aging also proposed the use of HRV as a way to predict heart attacks, although it concluded further research was needed.
The ways to interpret HRV may seem amorphous, but this is because it relates to one of the core functions of the human body. It’s an indication of the interplay between the sympathetic and parasympathetic nervous systems. One of these, the sympathetic, releases hormones to increase your heart rate, while the parasympathetic releases the hormone acetylcholine to lower it.
Exercise, or think about your bills, and your heart rate goes up — which you may think is no big deal given that your heart rate changes all the time, right? However, HRV can give some insight into how this interaction between the sympathetic and parasympathetic nervous systems is playing out.
A higher HRV suggests your cardiovascular system is more readily adaptable, and that’s a good thing.
How HRV is recorded
Using heart rate variability in your day-to-day life is not simple, though. Your HRV readings will change throughout the day, even if there’s no obvious stimulus to bring about such changes. And it will naturally be affected by stress and exercise, wherein your sympathetic nervous system grabs the wheel and causes your heart rate to rise.
Results will also vary between people, not solely linked to obvious factors like fitness, so you should really only compare your own results over time. Don’t fall into the trap of being jealous of your friend’s video game-style “high scores”.
Garmin uses it to feed into training recommendations, for example. “It gives you insight to make a clearer decision on how hard you want to go on your training. HRV Status feeds into the other metrics like training status, suggested workouts for even better feedback and guidance,” Garmin told us.
However, when we measure HRV we are also not interested in the rate of change of your heart rate during exercise — look to a metric like VO2 Max for that. Instead, HRV is typically recorded when there is no obvious stimulus that is going to skew the results, such as when you are asleep.
There are reasons your resting heart rate and estimated VO2 Max are typically more useful, and easier to understand, than metrics for the average person just looking to stay healthy. However, we are starting to see HRV scores given greater prominence in fitness tracker apps.
What is a “good” HRV?
The first thing you are going to wonder when you get your first HRV reading: “is that good? Is this bad? Am I dying?” It’s the wrong way to approach this stuff, but we reacted in exactly the same way.
Wearables maker Whoop published an interesting chart that shows how HRV stats tend to develop as we age. Much like VO2 Max, there’s a steady downward trend through your 20s and 30s, appearing to bottom out as you reach your 60s.
This graph shows the middle 50% of recorded results, discarding outlier high and low readings. At the age of 20, therefore, you might consider a normal range to be between 55ms and 105ms. At 40 this trends down to 36-64ms.
We should reiterate, though, that you could easily record HRV scores across these bounds — plus above and below them — in the space of a few hours.
Look for the trends in readings, rather than stressing over one result. However, several wearables makers make this easy to do in the way they present the data, and manage when they take recordings in order to limit the wild variances you can see in these variability scores.
HRV in the Apple Watch
Apple introduced HRV readings to the Apple Watch in WatchOS 4 back in 2018 and it remains available on their latest offering, the Apple Watch Series 7. They are taken automatically, in the background, at suitable times. However, you can also manually collect HRV data by starting a Breathe session in the Mindfulness app.
You won’t see the result when you finish the 1-minute breathing exercise, though. The data is ferried over to the Apple Health app. If you tap All Health Data from the app’s home screen you should see an entry for Heart Rate Variability.
This takes you to a screen that shows your HRV over time, with optional timescales of a day, week, month, 6-month period or a year. It’s a sensible way to relay the data, sitting outside of the regular fitness area and encouraging you to look at trends and averages, not single results.
HRV in Garmin watches
Garmin initially required you to use a chest strap for HRV readings, but not so anymore. With the release of 2021’s Garmin Venu 2 (one of the best Garmin watches) we were introduced to Health Snapshot, a 2-minute relaxation mode that spits out a bunch of stats including HRV.
You can find this in Garmin watches with the Elevate v4 heart rate reader tech, like the Venu 2, Fenix 7 and Garmin Epix 2. Some older watches, like the Fenix 6, measure HRV in their stress test modes too.
Garmin opened up heart rate variability recording significantly in 2022. The Forerunner 955 and Forerunner 255 introduced HRV Status, which takes readings overnight and shows you the results as part of a “morning report”. It is a much more friendly and passive way to try out HRV monitoring. We hope to see these features come to other Garmin watches, like the Fenix 7, in future.
HRV in Fitbit watches
Readings are taken overnight, possibly spurring Garmin to bring us its HRV Status mode a year later. We haven’t found Fitbit’s HR accuracy to be quite as good as Apple’s, or that of the latest Garmin watches. But part of a good HRV algorithm deals in discarding faulty readings, so this may not impact Fitbit’s scores.
There is an issue, though. Fitbit keeps long-term stat monitoring behind a Fitbit Premium paywall. It’s a subscription service that also unlocks guided workouts and nutrition plans. HRV stats are not particularly useful if your data only extends back a week, so if you want to monitor variability effectively you’ll need a Premium sub.
HRV in Samsung watches
Samsung uses HRV in its Galaxy Watches, but does not currently let you see the data. It feeds into stress readings, much like Garmin's old approach.
Third-party apps that claim to measure HRV are available from Samsung’s Galaxy Store, but we can’t speak to their accuracy. Still, if you’re desperate to try out HRV tracking, it is at least somewhat possible with a watch like the Galaxy Watch 4.
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Andrew Williams is a freelance journalist based near London. He has written about tech for over a decade, contributing to sites such as WIRED, TechRadar, TrustedReviews, Wareable, Stuff, T3, Pocket-lint and many others. When he's not covering fitness tech, he writes about mobile phones and computing, as well as cameras.