This number can tell you a lot about your overall health, but most people don't know what it means.

By Gabrielle Kassel
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If you rock a fitness tracker like festival-goers rock metallic fanny packs during Coachella, chances are you've heard of heart rate variability (HRV). Still, unless you're also a cardiologist or professional athlete, chances are you don't know what the heck it actually is.

But considering heart disease is the leading cause of death among women, you should know as much as possible about your ticker and how to keep it healthy—including what this number means for your health.

What Is Heart Rate Variability?

Heart rate—a measure of how many times your heart beats per minute—is commonly used to measure your cardiovascular exertion.

"Heart rate variability looks at how much time, in milliseconds, passes between those beats," says Joshua Scott, M.D., a primary care sports medicine physician at Cedars-Sinai Kerlan-Jobe Institute in Los Angeles, CA. "It measures the variation in the amount of time between those beats—usually aggregated over days, weeks, and months."

Interestingly enough, even if your heart rate is the same in two separate minutes (so the same number of heart beats per minute), those beats may not be spaced out in the same way.

And, unlike your resting heart rate (where a lower number is generally better), you want your heart rate variability to be high, explains cardiologist Mark Menolascino M.D., author of Heart Solution for Women. "Your HRV should be high because, in healthy individuals, the variation of heartbeats is chaotic. The more fixed the time is between beats, the more prone to disease you are." That's because the lower your HRV, the less adaptable your heart is and the worse your autonomic nervous system is functioning—but more on this below.

Think about a tennis player at the start of a volley: "They're crouched like a tiger, ready to move side to side," says Dr. Menolascino. "They're dynamic, they can adapt to where the ball goes. You want your heart to be similarly adaptable." A high variability indicates that your body can adapt to a given situation in a moments notice, he explains.

Essentially, heart rate variability measures how quickly your body can go from fight-or-flight to rest-and-digest, explains Richard Firshein, D.O., founder of Firshein Center Integrative Medicine in New York City.

This ability is controlled by something called the autonomic nervous system, which includes the sympathetic nervous system (flight or fight) and the parasympathetic nervous system (reset and digest), explains Dr. Menolascino. "A high HRV indicates that you can toggle back and forth between these two systems very quickly," he says. A low HRV indicates that there's an imbalance and either your flight-or-fight response is kicked into overdrive (AKA you're stressed AF), or that it's not working optimally. (See More: Stress is Actually Killing American Women).

One important detail: Research shows that arrhythmia—a condition when your heartbeat becomes too fast, too slow, or has irregular beats—can result in short-term HRV changes. However, true heart rate variability is measured over weeks and months. So a very high HRV (read: super variant) isn't indicative of something bad. In fact, the opposite is true. A lower HRV is associated with high-risk arrhythmia, while a high HRV is actually considered, ‘cardio protective’ meaning that it helps protects the heart against potential arrhythmias.

How to Measure Your Heart Rate Variability

The easiest—and, TBH, only really accessible—way to measure your heart rate variability is to wear a heart rate monitor or activity tracker. If you wear an Apple Watch, it'll automatically record an average HRV reading in the Health app. (Related: The Apple Watch Series 4 Has Some Fun Health and Wellness Features). Similarly, Garmin, FitBit, or Whoop all measure your HRV and use it to give you information about your body's stress levels, how recovered you are, and how much sleep you need.

"The reality is, there are no robust research studies in this particular area of smartwatches, so, consumers should be cautious about their accuracy," says Natasha Bhuyan, M.D., a One Medical Provider in Phoenix, AZ. That said, one (very, very small) 2018 study found that HRV data from the Apple Watch is pretty accurate. "I wouldn't hang my hat on this," though, says Dr. Scott.

Other options for measuring your heart rate variability include: getting an electrocardiogram (ECG or EKG), which is usually done in a doctor's office and measures the electrical activity of your heart; a photoplethysmography (PPG), which uses infrared light to detect subtle changes in your heartbeats and the time between those beats, but is usually only done at a hospital; and pacemakers or defibrillators, which are really only for people who already have or had heart disease, to automatically measure heart rate variability to keep tabs on the disease. However, since most of these require going to the doctor, they're not exactly easy ways to keep tabs on your HRV, making a fitness tracker your best bet.

Good vs. Bad Heart Rate Variability

Unlike heart rate, which can be measured and immediately declared, "normal", "low", or "high", heart rate variability is really only meaningful in how it trends over time. (Related: What You Should Know About Your Resting Heart Rate).

Rather, every person has a different HRV that is normal for them, says Froerer. It can be affected by a wide range of factors such as age, hormones, activity level, and gender.

For that reason, comparing heart rate variability between different individuals doesn't mean much, says Kiah Connolly, M.D., a board-certified emergency medicine physician at Kaiser Permanente and health director with Trifecta, a nutrition company. (So, no, there's no ideal HRV number.) "It's more meaningful if it's compared within the same individual over time." That's why experts say, while an ECG is currently the most accurate technology available for measuring HRV in the moment, a fitness tracker that's regularly gathering data and can show your HRV over weeks and months is best.

Heart Rate Variability and Your Health

Heart rate variability is a great indicator of overall health and fitness, says Froerer. Even though your personal HRV changes are the most important to keep an eye on, generally speaking, a "high HRV is associated with increased cognitive function, the ability to recover faster, and, over time, can become a great indicator of improved health and fitness," she says. On the other hand, a low HRV is associated with health conditions such as depression, diabetes, high blood pressure, and increased risk of coronary heart disease, she says.

Here's the thing: While good HRV has been tied to good health, research hasn't looked at sophisticated HRV patterns enough to make concrete cause-and-effect statements about HRV and your health, says Dr. Menolascino.

Still, heart rate variability is, at least, a good indicator of how stressed you are and how well your body is handling that stress. "That stress can be physical (such as helping a friend move or completing a very workout) or chemical (like increased cortisol levels from a boss yelling at you or a fight with a significant other)," explains Froerer. In fact, HRV's relationship to physical stress is the reason why it's considered a useful training tool by athletes and coaches. (Related: 10 Weird Ways Your Body Reacts to Stress)

Using Heart Rate Variability for Fitness Performance Insights

It's common for athletes to train specifically in their heart rate zone. "Heart rate variability is an even more in-depth look at that training," says Dr. Menolascino.

As a general rule, "People who are less trained will have lower HRV than people who are more trained and regular exercisers," says Dr. Scott.

But HRV can also be used to show if someone is over-training. "HRV can be a way to see the level of one's fatigue and ability to recover," explains Froerer. "If you are experiencing a low HRV upon waking up, that is an indicator that your body is overstressed and you need to lower the intensity of your exercise that day." Similarly, if you have a high HRV when you wake up, it means your body is feeling good and ready to get after it. (Related: 7 Signs You Seriously Need A Rest Day)

That's why some athletes and coaches will use HRV as one of many indicators of how well a person is adapting to a training regimen and the physiological demands placed upon them. "The majority of professional and elite sports teams are using HRV, and even some collegiate teams," says Jennifer Novak C.S.C.S. owner of PEAK Symmetry Performance Strategies in Atlanta. "Coaches can utilize players' data to adjust the training loads or implement recovery strategies to support balance in the autonomic nervous system."

But, you don't need to be elite to use HRV in your training. If you're prepping for a race, trying to place in the CrossFit Open, or just starting to go to the gym regularly, tracking your HRV may be beneficial in helping you know when you're going too hard, says Froerer.

Improving Your Heart Rate Variability

Anything considered good for your overall health—managing your stress levels, eating well, sleeping eight hours a night, and exercising—is good for your heart rate variability, says Dr. Menolascino.

On the flipside, being sedentary, lack of sleep, excessive use of alcohol or tobacco, long periods of heightened stress, having poor nutrition, or gaining weight/being obese can all result in a downward-trending HRV, says Dr. Menolascino. (Related: How to Turn Stress Into Positive Energy)

Do you need to monitor your heart rate variability? No, not necessarily. "It's good info to know, but if you're exercising already and otherwise optimizing your health, chances are your HRV is on the high side," says Sanjiv Patel, M.D., a cardiologist at MemorialCare Heart & Vascular Institute at Orange Coast Medical Center in Fountain Valley, CA.

Still, it might be useful if you're motivated by data. For instance, "having the data readily available can be a helpful reminder for CrossFit athletes not to over-train, for parents to be calm around their kids, or for CEOs in high-pressure situations to breathe," says Dr. Menolascino.

The bottom line is that heart rate variability is just one more helpful tool for measuring your health, and if you're already wearing an HRV-capable tracker, it's worth taking a look at your number. If your HRV starts to trend down, it may be time to see a doc, but if your HRV starts to improve you know you're living well.

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