Heart rate variability (HRV) as a term isn’t new — physicians have known about the phenomena since the 1700s. HRV is simply a measure of the variation in the time interval between heartbeats. You may think your heart beats like a metronome with the time between beats being equal. In actuality, the interval between heartbeats varies. Measuring the amount of variation can give you an idea about the state of your autonomic nervous system, which I’ll explain later.
So why is HRV becoming the hottest buzzword in fitness? The biggest names in fitness trackers (Apple, Whoop, Oura, Fitbit, and Garmin) are realizing HRV is somewhat simple data to collect, and consumers are obsessed with any information that can predict the effects of stress, illness, training, and sleep on their overall health. As a result, they are investing more in providing HRV information you can use.
When it comes to exercise, athletes who’ve often felt the pull of wanting to train while also feeling tired and run down can use HRV to take the guesswork out of knowing when to push hard and when to pull back. A low HRV score generally means your body struggles to recover from stress, while a higher HRV score indicates you’re responding well.
As of December 2020, Google search terms for “what is a good HRV range” and “good HRV score” grew by more than 5,000% over the previous time period. So if you’re going to start hearing more and more about HRV, it pays to know exactly what HRV is telling you, as well as what it’s not.
What HRV is Telling You
To understand HRV, you first have to take a short biology lesson, which I’ll try to make as concise as possible. A rested, healthy heart will have a certain amount of variation in the length of time between beats. This variation is controlled by the autonomic nervous system (ANS), which keeps our heart beating, our lungs breathing, and our food digesting.
There are two parts to the ANS — sympathetic and parasympathetic. The sympathetic nervous system is famous for the “fight or flight” response, while the parasympathetic system regulates our relaxation response, aka “rest and digest.”
Too much stress can put our sympathetic response into overdrive. This can include anything from a poor night’s sleep to a fight with your spouse to a super hard training session. In general, the more stressed you are, the less variation you’ll have between heartbeats.
When your ANS is healthy, you can bounce back from stress faster. Say you have a long run on Sunday, but with a few healthy meals and a good night’s sleep, your body is ready to train again the next day. The faster your body can maintain a balance between the sympathetic and the parasympathetic systems, in general, the more resilient and healthy you are.
Most people believe the higher your HRV score, the better. However, it’s actually best to have an HRV score that remains relatively stable. Each app calculates HRV differently, but I’ll use an example from the HRV program I use (ithlete). My score averages around 85 (this is an output devised by the program and is not a measure of time between beats).
If I wake up and my HRV is 65, I’ve likely experienced a highly stressful 24 hours (for whatever reason) and may need to take a day off from training or plan to go bed earlier. But if I wake up and my HRV score is 105, my parasympathetic system is in overdrive. I may think the unusually high score is an indication to push hard, when in fact, my body is crying out for rest.
Large swings up or down from average are something to pay attention to. I find when my parasympathetic nervous system is in overdrive, I feel ridiculously tired and unmotivated.
You can also track changes in your average HRV over time. If you’ve started a new diet or fitness regimen, you can use daily HRV readings to see how your body responds. Your average score should increase over time.
What HRV Is Not Telling You
While fitness devices are making HRV information more accessible, it’s important to remember interpreting that data isn’t always easy. HRV is highly susceptible to any changes in sleep, diet, stress, and a large number of other factors.
Shawn Arent, chair of the Department of Exercise Science at the University of South Carolina, notes,
Let’s say an athlete’s HRV is low but she actually feels good. Maybe the low score is because she ate dinner late the previous night. Should that guide what she does that day? We don’t know really what’s causing that low number. Using HRV by itself can give a misconception. It’s a useful tool but not as a stand-alone.
The more you examine your HRV data, the better you’ll get at interpreting the result. For instance, I’ve found my HRV can take a nosedive after a heavy meal or a second glass of wine before bed, but I can still have an energetic and productive training session. I also know that if I’m developing a cold, experiencing high work stress, and feeling run down, my body would recover faster by taking it much easier in the gym or adding a rest day regardless of my HRV score.
Remember, HRV can tell you the state of your ANS at that moment in time. Sometimes stress can take a while to accumulate to a level where it reflects in your score. I’ve measured HRV daily for more than four years, and there are still moments where the outcome surprises me.
Use HRV as one tool, but not the only tool. Listening to your body and using common sense is still important.
How is HRV Measured?
If you’re getting an HRV measurement from Apple Watch, Whoop, Oura, or any other fitness apps, the best thing you can do is go to their websites and search for information on how they collect and report their data. Each one may be slightly different, and it would be too cumbersome to explain them all here.
It’s important to know the only truly accurate way to measure HRV is to use an electrocardiogram in your doctor’s office. However, technology accessible to average consumers is constantly improving. The easiest way to measure HRV (in my opinion) is to use a chest strap heart monitor and Bluetooth it to any one of several apps that collect data (Elite HRV and ithlete are both good ones).
These apps have you take a measurement first thing in the morning before fully waking up and consuming caffeine. They give you a “score” and help you interpret what the score means.
Other trackers like Apple Watch will give you a reading in milliseconds (ms). But they are measuring your HRV at random points during the day (using a time-domain method) or, as in the case of Oura, overnight. A study in Frontiers in Public Health states,
Heart rate variability time-domain indices quantify the amount of HRV observed during monitoring periods that may range from <1 min to >24 h. These metrics include the SDNN, SDRR, SDANN, SDNN Index, RMSSD, NN50, pNN50, HR Max − HR Min, the HRV triangular index (HTI), and the Triangular Interpolation of the NN Interval Histogram. Time-domain measurements rise with increased aerobic fitness . In general, HRV time-domain measurements decline with decreased health .
That said, if you are interested in knowing how your HRV compares with other people’s, a paper published by The Journal of the American College of Cardiology found the average SDNN for someone in their 20s was 72ms. In comparison, in their 30s, this went down to 64ms.
I’m confident you’re going to hear more and more about HRV in the coming months to years. How confident? I’d bet money in Vegas if that were a thing.
As you become more aware of HRV as a marker of the state of your ANS, there are a few important things to keep in mind.
- HRV is measuring the state of your ANS at a given point in time. It will take several weeks to establish a baseline average. After that, large swings up or down indicate a hyperactive fight or flight (sympathetic) response or hyperactive rest and digest (parasympathetic) response. Small ups and downs are generally not worth paying attention to.
- While higher HRV measurements indicate better overall health, you should measure how your HRV average changes over time. Day-to-day changes are less reliable, as HRV is highly sensitive to several stressors, and not all stress is bad (think exercise).
- Use HRV as one tool to monitor your ability to recover from stress. Also, listen to your body and pay attention to other health markers like blood pressure, blood sugar, and cholesterol.
- Look to your particular tracker or app for more guidance on interpreting their results, as everyone is slightly different.
These pointers are meant to highlight the overall usefulness of HRV and give a broad overview of what it means. I’m excited to see how the technology evolves in the coming year, as it will only grow in popularity.