What Is Heart Rate Variability (HRV) on Apple Watch, and Why Does It Matter?

Heart rate variability (or HRV) is the latest hot health measurement. It’s being used by athletes to optimize their training, and as a predictor of health—including in a number of studies on COVID-19. Your Apple Watch tracks it automatically. Here’s what you need to know about it, and why it matters.

The author of this article is not a doctor. This is a technology article explaining a feature of the Apple Watch. If you have any concerns about your health, contact your physician.

What Is HRV?

Heart rate variability is a measure of how much the time between two heartbeats varies from beat to beat. It’s controlled by your autonomic nervous system (ANS), which is responsible for breathing, digestion, and other unconscious bodily functions.

The ANS has two big subcomponents: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The SNS reacts to stress and controls your body’s “fight-or-flight” responses. The PNS, on the other hand, controls your body’s “feed-and-breed” and “rest-and-digest” responses.┬áIt’s what ticks over when everything is normal.

HRV is a way of measuring ANS activity. When the SNS is active—in other words, when you haven’t slept too well, are fighting with your partner, or are otherwise a bit stressed—the variation between heartbeats is lower than when the PNS is active. If you’re relaxed and chilled out, your heart rate just bounces around a bit more.

Of course, stress isn’t the only thing that affects HRV. Age, gender, genetics, hydration, exercise, illness, and many more things influence it. Stress is just one of the big factors that can do it.

What Is a Good HRV Measurement?

There are no absolutes when it comes to HRV, only individual trends.

In general, a higher HRV indicates that you are more relaxed and less stressed than a lower HRV does. However, what is an abnormally low HRV for one person can be a high one for someone else. Whoop, a manufacturer of heart rate trackers, has a useful graph on its site showing the middle 50% of values for people at different ages: It’s quite a large range.

Instead, what you’re looking for is changes in HRV. If your HRV drops lower over a few days, you might be more stressed than normal, not sleeping well, or even getting sick. Or, you could just be working out a bit too hard and needing to take a day or two off.

How Accurate Is the Apple Watch’s HRV Measurement?

According to a study comparing the Apple Watch to a dedicated heart rate tracker, it’s pretty accurate. An ECG from your doctor will always be the gold standard, but you can probably assume that your Apple Watch is not totally off base, at least as far as broad trends are concerned.

How to Check Your HRV With Your Apple Watch

Your Apple Watch automatically tracks your HRV while it tracks your heart rate throughout the day. There are a few different ways to measure the variation between beats, but the Apple Watch uses SDNN, which gives a value in milliseconds.

To view the data, open the “Health” app on your iPhone and go to Browse > Heart > Heart Rate Variability.

health app heart highlighted

health app HRV highlighted

Here, you’ll see a graph of your HRV. Tap “D” to see the daily measurements, “W” to see averages across a week, “M” to see them across the past month, and “Y” to see the trends over the last year.

HRV graph

There are also third-party apps, like Welltory, that can help you dig deeper into the data.

HRV is an increasingly popular health measurement, but it’s still quite esoteric, and there’s lots of research yet to be done on it. We’ve been very wary about linking out to any sources making bold claims about the benefits of religiously monitoring it.

For now, your Apple Watch tracks it but does little else with it. That may change in the future. Still, you can keep an eye on your HRV and see how it varies over time with different activity and sleep levels as you continue about your daily life.

Of course, if you’re at all concerned about what you’re seeing, contact your doctor immediately.

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