What Is Heart Rate Variability? Here’s Everything You Need to Know About Fitness’s Buzziest Metric

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Photo: Getty Images/Credit: AsiaVision
Top athletes have long turned to heart rate variability as a go-to data point to track performance and recovery. As advances in fitness have made what was once reserved for gold medalists available to all (Theragun! Normatec! Infrared pajamas!), the technology trickle down has helped many everyday fitness enthusiasts train smarter, recover better, and perform more consistently.

Heart rate variability (HRV)—the differentiation in timing between two heartbeats—is captured by wearable devices like an Apple watch, Fitbit, WHOOP band, or Oura ring. According to WHOOP founder Will Ahmed, it's "one of the best objective metrics for physical fitness and determining your body’s readiness to perform."

As the wearables market has exploded (we reported in our 2020 Wellness Trends that from 2018 to 2022, the category is expected to double, for a whopping net worth of 27 billion dollars), HRV metrics have been pushed onto the wrists of millions of users. So, it's worth a closer look at what HRV indicates and how to use it to better your workout... and your life.

What is heart rate variability, and why should we care about it?

Your pulse tells you how many times your heart beats every minute, but it doesn't necessarily tell you the cadence of those beats. "It isn't a metronome," says Emily Capodilupo, WHOOP's VP of data science and research. Instead, it could beat once, then again after 0.95 seconds, and then again after 1.12 seconds. As such, HRV measures the changes in milliseconds between successive heartbeats. The more of a difference there is in the timing between two beats, the higher your HRV.

While it seems as though this would be a marker of cardiovascular fitness, your HRV is actually dictated by the autonomic nervous system, which is responsible for regulating involuntary responses within the body such as digestion, temperature, and (surprise!) heart rate.

The autonomic nervous system is made up of two branches: the sympathetic nervous system and parasympathetic nervous system. The sympathetic nervous system releases hormones to speed up your heart and prepare your body for stressful "fight-or-flight" situations, whereas the parasympathetic nervous system releases hormones to slow the speed of your heart.

Your HRV is an indicator of how these systems are working in tandem. It "tells you how quickly you can increase your heart rate when you put demand on your heart, and how quickly it relaxes when you stop exercising," says Nate Favini, MS, MD, medical lead at Forward.

What does it mean to have high or low HRV?

When everything is functioning properly, these two branches of the autonomic nervous system will be balanced and your body will be able to properly respond to various stimuli. "When your heart rate variability is high, it means your body is listening to both branches of the autonomic nervous system, and it’s a sign of being responsive to your environment and being able to adapt to changes in it," says Capodilupo.

In general, pros say it's a good thing to have high HRV, because it means your parasympathetic nervous system is relaxed, and your heart is highly responsive to changes in your activity. "People with higher heart rate variability are spending presumably more time in that relaxation phase, so they’re not chronically stressed with a chronically high heart rate," says board-certified cardiologist Jen Haythe, MD.

But sometimes, you may find yourself trying to figure out how to slow down your heart rate. When your parasympathetic nervous system is out of balance—which can happen thanks to things like stress, being sick, sleep deprivation, poor diet, and lack of exercise—it can present in a low HRV reading. These things place your body on high alert and kick your "activating" fight-or-flight response into high gear.

"If you're constantly activated, it diverts resources away from your body's maintenance processes like sleeping, turning over the cells in your hair skin and nails, and maintaining your immune system in order to focus on more immediate processes," says Capodilupo. "If you’re not able to respond to both of those types of needs at the same time, you wind up short-changing the one you’re not listening to."

What can heart-rate variability tell you about your health?

In a phrase: a lot. Low HRV in general is linked to mental health conditions like stress, depression, and anxiety, as well as physical conditions such inflammation, chronic pain, diabetes, concussion, asthma, insomnia, fatigue. What's more, research has directly connected a decline in heart rate variability with declining health.

Your HRV can even be used as an indicator that you might be getting sick. "When first get an infection, the bacteria [or virus] is growing inside your body, but you feel totally fine and won't show symptoms because your nervous system is diverting resources to your immune system to fight that infection," says Capodilupo.

During this incubation period, your average HRV will drop for a few days in a row before you actually start to show symptoms. "That’s a really powerful tool, because during that time you can intervene to support your immune system while you still feel really good, and do things like hydrate and rest, and avoid additional stressors on the body like alcohol or super hard workouts," she says.

What is a "good" HRV metric?

While doctors can provide you with general guidelines of what high HRV looks like for your age and sex demographics, it's more important to focus on your own HRV, as it's tracking over time versus what your HRV looks like compared with someone else.

"If you have an average HRV of 60 ms [(milliseconds)], that’s not necessarily better or worse than someone who’s average HRV is 80 ms," explains Capodilupo. But if your average HRV is 60 ms, and it suddenly drops down to 40 ms for a few consecutive days, it could be a sign that something is wrong. Or, if it suddenly jumps to 80 ms, it means you're taking good care of your body.

If you're interested in improving your average HRV, there are a few lifestyle changes that can help. Meeting your basic needs like getting enough sleep and hydration, managing stress, eating a healthy diet, and staying away from alcohol will all have a positive impact, and keep you moving at peak performance.

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