Unlike your resting heart rate, which you can take with nothing more than a stopwatch and a finger to your pulse, tracking your heart rate variability (HRV) is more complicated. While it used to require getting hooked up to an EKG at your doctor’s office, new smartwatches and other wearables have made your HRV another stat that’s easy to track—and some top athletes and coaches are tapping into it to fine-tune their workouts.

“HRV is one more piece of data that can potentially be helpful, but it could also be confusing since so many factors can affect it,” says Lili Barouch, M.D., director of sports cardiology at Johns Hopkins Medicine, who’s an avid cyclist and Ironman triathlete. “It isn’t as easy to understand as heart rate or sleep cycle information, and the reason for a change in HRV isn’t always obvious.”

The ups and downs of HRV can be so tricky to navigate, in fact, that most doctors, even cardiologists, don’t factor it into treatment plans for their patients. So is it a metric you should start tracking—or one of the bells and whistles on your smartwatch that you should maybe skip over?

Here’s everything you need to know about heart rate variability and why it’s so complicated—plus how doing a deep dive into yours could potentially help improve your performance on the bike.

What is heart rate variability and what should it be?

You know that heart rate is beats per minute and you’ve likely looked at your resting heart rate, which is your reading at rest, ideally when you wake up in the morning and haven’t gotten out of bed yet. HRV digs in much deeper to see what’s happening between beats.

“Heart rate variability is a measure of the slight beat-to-beat changes in heart rate, usually expressed in milliseconds, or 1000ths of a second,” says Barouch. “In general, higher HRV is better and indicates better health, better fitness, or training readiness, while lower HRV usually indicates physical or mental stress on the body.”

That stress can stem from a lot of sources, like having a cold, drinking a few cocktails, getting too little zzz’s, or being anxious about a looming work deadline. (Studies have linked daily stress to lower HRV.) Because heart rate variability rises when you’re in more a of “rest and digest” mode, it can be sensitive to a range of stressors, which ultimately lower the number.

There’s always going to be some amount of fluctuation in your heart rate variability—variability is in the name, after all—and that doesn’t mean that you should have any concern about an arrhythmia or other potential underlying health issue (though some arrhythmias could cause a sudden spike in HRV).

The question of what your HRV should be is much more complicated. First of all, there’s a wide range that’s considered okay: An adult’s score can fall anywhere from under 20 to over 200 and still be considered normal.

“Everyone’s range is different. For me to compare my HRV to your HRV probably isn’t appropriate because of all those various factors that affect HRV,” says Michael Emery, M.D., past chair of the American College of Cardiology’s Sports & Exercise Council and codirector of the Cleveland Clinic Sports Cardiology Center. “Instead, it’s more important to establish a relatively long baseline and then examine how your HRV changes from that baseline.”

By “relatively long,” he means a couple months, which is one of the reasons it’s difficult for clinicians to use HRV when treating patients.

How do you track your heart rate variability?

As mentioned, tracking HRV used to require being hooked up to the electrodes of an EKG in a doctor’s office. And some chest straps still use a similar electrode-based technology, so they can track heart rate very accurately. But they aren’t comfortable to sleep in—so other wearable devices, from smartwatches to rings, make more sense for following your HRV. Instead of electrodes, these other wearables effectively gather HRV data using a light-emitting technology.

“How you calculate it is a little bit different from system to system,” explains Emery. “Some devices do it during the nighttime, some do it first thing in the morning, and that results in statistics that are a little bit different. It makes comparing from device to device a little difficult.” (That difference in modes of tracking is another reason why it doesn’t make sense to compare your HRV to anyone else’s.)

Keep tabs on your HRV for at least four to eight weeks before you try to read anything into it, advises Emery. And it’s not just about glancing at the number on your wrist each morning. “As you’re starting to collect what your baseline HRV is, it’s important to keep a diary along with it,” he says. “Track everything from your caffeine intake to your sleep and hydration level—all the things you know could affect it. After you have your HRV for that period of time, along with all the notes about those HRV-perturbing factors, that’s when you can start to learn and use it more.”

Other factors to keep in mind that might affect heart rate variability: stress, exercise levels, illness, diet, and emotions.

Keep in mind, some trackers will do this recording for you, giving you warning when your HRV changes dramatically.

Additionally, it’s ideal if nothing out of the ordinary happens during the time you’re tracking your baseline heart rate variability—i.e., if you come down with COVID or have a two-week European vacation planned (even a change in time zone can affect your number!), it’s better to reset your tracking diary and start collecting another month or two of data.

Why does heart rate variability matter for athletes—and how can it inform your training?

HRV can be very helpful in training because a higher HRV usually indicates good “training readiness,” says Barouch. For example, if you notice your HRV is on the high end of your average, you might increase the intensity or duration of your upcoming workout.

On the flip side, if your number is dipping lower than usual not just one day but consistently, it could indicate that you’ve been pushing it too hard and need to dial things back. “Paying attention to these metrics may help reduce the risk of injury or overreaching because a persistently lower HRV can be an early sign of overtraining and could indicate to an athlete or their coach that a change in training might be needed,” says Barouch.

She notes again that other factors, like dehydration or tiredness, could be the actual culprit bringing down your number, so it’s not always overtraining, but it’s worth a look at your schedule to see if you need to cut back.

These are some of the ways elite athletes and coaches have started implementing HRV in their training schedules. In fact, using HRV to plan daily workouts has the potential to improve peak power output compared to planning workouts based on periodisation, according to a 2018 study published in the International Journal of Sports Physiology Performance.

“Athletes of that caliber are always looking for that very small edge, so some elite-level coaches are using this data,” says Emery. “For your average person, using HRV in this way depends on how interested you are in figuring out what small choices throughout your day affect your HRV—and if you have the flexibility to change your workouts to different days of the week depending on whether your number is higher or lower than your baseline.”

Barouch agrees: “Again, it is only a guideline and has a fair amount of variation from day to day, so it is probably better used for looking at trends rather than everyday changes to a training program.”

The simplest way for an amateur cyclist to think of HRV as a training tool, Emery says, is to focus on healthy decisions throughout your day—limiting caffeine, talking with a therapist or meditating to reduce anxiety, eating a nutritious diet, and prioritising sleep. All these incremental actions can help boost your HRV and that, in turn, can help bump up your performance on the bike. You can play around with these factors, see how they affect HRV, and choose which ones bump up the number the most, so they become a part of your lifestyle.