Heart rate variability and wearables explained

A built-in heart rate sensor is something we take for granted these days when we talk about wearables.

Every smartwatch, fitness tracker, and sports device has a built-in heart rate monitor, used for 24/7 monitoring and tracking workouts. But increasingly, heart rate variability (HRV) is a key metric.

HRV is a heart rate measurement that the likes of Fitbit, Garmin, Apple, Polar Samsung, and others use to offer insights such as stress, recovery, readiness, and fatigue. In fact, of the new era of health insights on offer from wearables, HRV is at the heart of them all.

Here's what it means and why it's so important.

• Best heart rate monitors to buy

What is heart rate variability?

Heart rate variability is the measurement of the time interval between heartbeats.

Unlike measuring heart rate, which is about the average number of heartbeats per minute, HRV focuses on the small fluctuations of the heartbeat and usually measures that variation time in milliseconds. The higher your HRV, the better.

Those fluctuations can be affected by a whole host of things.

That could be age, body position, the time of day, and health status.

Then there's a whole host of mental, physical, and emotional experiences that can have an impact on HRV measurements and data. A lot can impact your HRV.

The benefits of tracking HRV

Why is it useful to measure or monitor it? Well, for fitness particularly, it's a way of knowing when you're putting too much stress or strain on your body and can be an indicator of whether you're physically and mentally ready to work out again.

So if you're a runner and you've just put in a tough 2-hour run, it'll be able to help you decide whether you're ready to go again the following day or whether you should take the day off from training. When you're working out, your HRV decreases as your heart rate and exercise intensity rise.

But it's not just about serious athletes, as HRV measurements have started to trickle down into devices like fitness trackers which it can also be used to check in on how stressed you are.

What is a normal, high, or low HRV?

So what does it mean to have a low or high HRV reading? There's no ideal HRV score, and like resting heart rate, it's hugely personal.

But generally speaking, high HRV is associated with healthy longevity and the side of the nervous system that promotes relaxation, digestion, sleep, and recovery.

A low HRV reading is commonly associated with stress, overtraining, inflammation, and illness. Diabetes, coronary heart disease, and high cholesterol are things commonly tied to having a low HRV as well.

What is a high, low, or normal HRV reading? A blog post on that subject by Oura, makers of the Oura Ring smart ring, which measures HRV during sleep to assess your readiness, explains:

"High and low HRV is relative for each person. HRV is a highly sensitive metric, which responds uniquely for everyone.

"Some individuals have steady HRV scores, while others fluctuate greatly. HRV is an evolving tool, which means, at every HRV level, your personal scores and body status observations are especially important."

While Oura has calculated what its research and studies interpret as a normal HRV, it makes clear that every person's HRV is unique and you should look to compare your averages as opposed to others to make the best sense of the data.

Wearables and HRV:

As we said, most major wearable makers are tapping into HRV measurements for a host of insights.

We've broken down how the key players are using the data to fuel features on your smartwatch or fitness tracker.

Apple Watch (all versions)

Wareable

HRV is a staple feature of the Series 9, Ultra 2 and Apple Watch SE.

The way Apple treats HRV tracking natively is by continuously recording it with its optical heart rate sensor during your wearing time and letting you view trends inside of the Apple Health app on your paired iPhone. From the Health app, you can see trends over the day, week, month, and year to see where there have been increases and decreases in HRV measurements.

If you want to delve deeper into what the measurement can tell you, there are third-party HRV Apple Watch apps like HRV4Training that can help you interpret the data and offer more actionable data.

Fitbit

Wareable

Most new Fitbit fitness trackers and smartwatches offer the ability to measure heart rate variability and pay close attention to those variations between heartbeats mainly during sleep.

You'll find it on everything from the Fitbit Inspire 3 to the Sense 2.

Fitbit reports HRV directly to the Health Metrics section of the Fitbit app. From there, you'll be able to see the latest HRV measurement, which is measured in milliseconds (ms) from the longest sleep period over the past 24 hours.

Basic users of Fitbit can see the HRV trend over the past 7 days, but Premium users can track that for 30 and 90 days.

If you see a significant decrease in HRV, it could be a potential sign of major stress, illness, or fatigue. It's worth mentioning that Fitbit this feature isn't intended for medical purposes.

In addition to simply measuring HRV, that metric is used as part of Fitbit's Daily Readiness Scores to guide you on whether you're in good shape to train hard or whether you should take a day off.

Whoop

Wareable

HRV also unerpins the Whoop experience, and this exccellent fitness trackwer puts a lot of weigh on the heart rate measurement.

First is the Recovery score, the all-important daily percentage of readiness that Whoop offer users. This is majorly weighted to your HRV, as an estimation of your body's status.

HRV is also presented in the Health Monitor. This dashboard of data shows a range of measurements, and your averagre daily baseline. You'll get a green status for anything within range or over, but if your HRV is significantly lower, you'll get an orange or even red warning.

Finally, HRV is a major part of Whoop's new Stress Monitor, which shows a live estimation of stress levels throughout the day.

Samsung

Wareable

On Samsung Galaxy Watch, heart rate variability measurements along with heart rate measurements are used to fuel the stress monitoring features available on the smartwatch, which can be done continuously and lets you see on-the-spot stress measurements directly on the watch.

Garmin

Wareable

Garmin has increasingly embraced HRV measurements across a range of its watches.

Most new watches (see Venu 3, Forerunner 265, Vivoactive 5) show HRV Status (shown above) which takes an average HRV reading from the night, and a 7-day average both compared to your baselines.

On its premium watches (such as Fenix 7 and Epix), Garmin interprets HRV as a Training Readiness metric. Much like Whoop and Oura, this gives you a score on how ready your body is for a tough workout – or a signal that you might need a rest day. 

For sleep, it uses HRV to improve the accuracy of determining when you've been awake and the time you've spent in the different sleep stages it's able to recognize. 

Wareable

Like Samsung, Garmin also uses HRV data to track stress levels.

From an exercise and training point of view, it's also a metric that fuels data such as your VO2 max, performance condition, and lactate threshold and it powers Garmin's Body Battery energy monitor too.

Polar

Wareable

Polar also uses HRV to help you better assess when to train and when to spend time recovering. Watches like the Polar Vantage V3 offer an Orthostatic test, which is based on HRV and requires an external heart rate monitor chest strap to conduct the test.

The test results fuel Polar's Recovery Pro features and if you're doing the tests regularly, it can help you better understand whether fatigue is training-based or based on another factor.

Oura

Wareable

We've already spoken a bit about Oura and its smart ring, which is a wearable designed to help you better understand your recovery needs.

It tracks HRV overnight in 5-minute samples. It uses those continual measurements to better inform how your body is responding to stresses and strain you put on the body on that day.

In its companion app, you can then view average HRV, max HRV, and HRV Trace which can be viewed in the Readiness tab and Trends section of the app. You can then use that data to understand how well you've recovered.

In addition to those, you can also explore HRV Balance, which looks at a longer-term comparison of your current HRV trend and your baseline to understand whether you're recovering well or putting your body under serious stress.

How accurate is HRV tracking and what's coming next?

So we've touched on this subject above, but it's worth returning to. To get reliable heart rate variability measurements, the sensors taking the readings need to be up the task. That's when we once again fall into this whole wrist versus chest heart rate debate.

When we spoke to biometrics experts Valencell, they told us, "It's really difficult to deliver heart rate variability from the wrist when you have so much movement of the wrist."

So much like most aspects of wearables data, HRV trends can be a useful window into our wellness. But relying on the numbers as absolute gospel can be subjective. Not only is it dependent on accuracy, but also how and when the HRV figure is taken and averaged.