A couple of months ago, I got a new Garmin watch. This one, unlike my last one, offers a “morning report.” This report begins with judgmental comments about my sleep (“Too short, not enough REM”), suggests a workout for the day, and ends by telling me my overnight HRV — heart rate variability.
Up until that point, I hadn’t really thought about HRV and had only a limited idea what it even was. But once I started paying attention, it seemed to be everywhere. Athletes I follow were tracking it. Someone I work with asked what a “good” number was. A friend told me she was using it to track her ongoing COVID symptoms. So I got curious — and if you’re also curious, today is your day.
I’m going to dive into various heart metrics, what they mean, why you might care about them, and whether you should be tracking them.
Overview: heart rates
Heart rate is a count of the number of heartbeats per minute (bpm). You can take yours right now, either by taking your pulse or, if you have a watch, looking at that. There are several different heart rate concepts floating around, which are useful to define before we get into health issues.
Maximum heart rate: The maximum number of beats per minute your heart can do. This is the heart rate you’d hit if you were doing all-out exercise. This number varies with your age and your genetics. A rule of thumb is that the value is 220 minus your age, although there is significant individual variation. This maximum heart rate value is not something you control, and it doesn’t say anything about your health. We will not speak of it again!
Resting heart rate: The number of beats per minute, on average, when you’re at rest. This can be well measured overnight, or if you sit quietly. Resting heart rate (RHR) is easy to measure, so we have a lot of data on what is typical. Based on a large study of U.S. adults, average resting heart rates are around 70 to 80, with most individuals being between the mid-50s and just over 100. Unlike maximum heart rate, resting heart rate is influenced by behavior — specifically, exercise. Trained athletes often have heart rates in the 40s or even lower (the lowest resting heart rate ever measured was in a cyclist — 27 bpm).
Heart rate variability: Although we talk about heart rate as if your heart is a metronome, a healthy heart actually has tiny variations in the length of time between heartbeats. HRV is a metric of these variations. Generally, higher HRV is a marker of health (more below).
HRV isn’t as easy to measure as resting heart rate — for one thing, you can’t measure it without technology, since the variations are far too small to be detectable. In addition, to track it over time in a useful way, it needs to be measured at about the same time and in the same conditions every day.
Part of the reason we are hearing so much more about it now is that smartwatches, Oura rings, the Whoop, etc. have made it possible for individuals to collect this data reliably. Our best population-level data on HRV levels comes from these devices — here’s a nice graph from Oura showing that HRV declines with age, and averages between the mid-20s and the 90s. Generally, watches and rings collect this information overnight as you sleep, hence their appearance in my morning report.
With these definitions in mind, let’s turn to the relationship between these heart rate measures and health.
Heart rates as health signals
There is a lot of evidence that a low resting heart rate and a higher average HRV are signals of health. Here (for example) is a meta-analysis of over a million individuals showing that a higher resting heart rate is associated with elevated mortality risk. Heart rate variability measures also clearly differ between healthy individuals and those with cardiac issues.
Even without direct data, cardiac biology would tell us that these are ways to measure healthy heart function, and we know that the heart is an important organ.
Although these are both signals of health, they are not per se causally related to health status. Here is what I mean: if we were able to produce a pill that magically slowed your heart rate but didn’t change anything else about you, there isn’t any reason to think that your health and longevity would be improved.
Instead, what is true is that activities and characteristics that affect these metrics are, themselves, important for health. The most obvious is exercise. There is excellent evidence, from randomized trials, that exercise lowers resting heart rate and can increase HRV. Exercise itself has been shown (again, in randomized trials) to improve many real health outcomes.
Bottom line: It makes sense to think about these heart rate measures as signals of health, and actions that you take to improve them probably also improve your health.
HRV vs. RHR
Now we get to the interesting stuff. On average, both HRV and RHR are metrics of heart health. Where they differ is in their day-to-day variation.
Your resting heart rate is broadly responsive to aerobic fitness. It can vary over a longer period — a training cycle, for example, or as you increase your exercise capacity. But it does not show a lot of day-to-day variation. In contrast, HRV can vary significantly day-to-day, based on many lifestyle factors.
This is convincingly demonstrated in an excellent paper that used non-experimental data from individuals to observe both resting heart rate and HRV changes over time in response to various things. As one example: the authors collected data on training intensity from participants. Immediately after high-intensity training days, resting heart rate varied by 1.3% but HRV varied by 4.6%. HRV is similarly about twice as responsive as RHR to the menstrual cycle, alcohol consumption, and illness.
In contrast, these authors found that average resting heart rate was more closely correlated with general metrics of health (BMI, for example) than average HRV was.
Putting this together: variations in HRV better track your day-to-day health. If you do any of this tracking, this will probably be familiar. In my case, last weekend was a clear example of life-meets-data.
On Saturday I had a hard workout (“overreaching” is the word Coach Kaitlin used) and then went to a wedding where I had two glasses of wine (this is one more than my appropriate limit). In contrast, on Sunday I did a little bit of cross-training and enjoyed an Athletic Brewing Lite non-alcoholic beer in the evening.
My judgmental watch tracks both resting heart rate and HRV overnight. On Sunday, relative to Saturday, my resting heart rate dropped 6% (51 to 48) but my HRV increased by 26% (53 to 67). (To be clear: I do not regret the second glass of wine! One must sometimes have fun.)
Your HRV will also respond to other life stresses — a big project at work, a problem with your spouse or kids, grief or loss.
Final note: Both resting heart rate and HRV respond to pregnancy. During pregnancy, resting heart rate goes up — as much as 10 to 15 beats per minute — and HRV on average goes down. These will both return to baseline fairly quickly postpartum.
Should I track these, and why?
You should have some general idea of your resting heart rate, especially if you have worries about your heart health. Day-to-day tracking of this heart rate isn’t likely to tell you much of anything.
With HRV, there are stronger arguments for daily tracking, because day-to-day variation actually does tell you something. This is not, however, useful for everyone. The question, as with any data, is what you are going to do with the information. A couple examples of reasons you might actually need to know this:
- You’re a serious athlete. Some (not all) athletes track their HRV to guide their training. There is some evidence that it could be optimal to target training to HRV status (basically, train more when you hit a good HRV day). More generally, a reduction in HRV can indicate overtraining.
- You are experimenting with some lifestyle changes and want another way to evaluate them. Because HRV has more of a real-time response, if you’re exploring lifestyle changes (say, cutting out alcohol or changing diet), tracking HRV could provide consistent feedback in a way that may make changes more sustainable.
Honestly, I wanted a third example here, but I couldn’t come up with it. For most people, continuously tracking HRV is probably unnecessary. It could be fun! I guess that’s the third reason — if you are a devoted personal-data nerd (no judgment; this is what will be on my tombstone), you might just enjoy it.
I will issue one caution, coming back to my watch. Yes, these numbers may have some relation to how you feel, either in exercise or in general. But they aren’t a crystal ball. You can have a good workout, or a good day, even if your tracking program tells you you’re tired.
I love data … but there are also times when we might do better to just ignore it.
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I am interested in knowing how much RHR can be genetic. It seems everyone in my family has low RHR although some people never exercise and have other health issues ( high blood pressure, obesity etc) but still have a RHR of 50 whereas some exercise a lot and have unusually low RHR in the 30s. Is there a genetic component to this?
Usually my Apple Watch just warns me that I have unusual BPM when I’m drinking alcohol so I have pretty much ignored it until last year. After having COVID, I started experiencing irregular resting heart rates and my Apple Watch helped confirmed it. It prompted me to go to get checked out with my PCP and after much testing and specialists, found out I have Graves Disease. I am now on daily medication to regulate my hyperthyroidism and am grateful for my watch.
I’ve been wearing a Whoop for a few years, including during pregnancy. Pregnancy wreaked havoc on my RHR and HRV as expected, but my numbers returned to normal postpartum and have improved since I restarted exercise. I’ve found that while you can improve your RHR and HRV through exercise, you won’t stray too much from your baseline. My baseline average RHR is somewhat high and my baseline average HRV somewhat low. They’ve improved with regular cardio and strength training, but still do not come near my husband’s stats. It would be interesting to see the research on baseline RHR/HRV and to what extent they can be improved.
Hi! Can you comment on studies related to HRV predicting labor timelines? WHOOP found that HRV and RHR trends flip 7 weeks. before childbirth. I’m trying to figure out how much stock to put in that 🙂 https://www.whoop.com/us/en/thelocker/pregnancy-study-exercise-hrv-rhr/
I used a Whoop while pregnant and didn’t find this to be true for me, although HRV and RHR did trend better closer to childbirth. I was disappointed because I was hoping to use it to predict when baby was coming!
I recently had a special experience with the RHR data tracked by my Apple Watch. My watch pinged me in Jan about a new trend in my RHR – it had increased by 10 bpm. Being 9 months postpartum I feared it was an indicator of declining fitness since I had not yet gotten back into exercising. So I bought a peloton!
But then I brought my RHR data to my annual physical the following month, and a blood test confirmed I had hyperthyroidism, which explained my symptoms of heightened anxiety and elevated RHR. I was able to begin treating my hyperthyroidism and was thrilled to get another “new heart rate trend” alert on my watch, confirming my RHR had decreased back to baseline and coinciding with relief of my anxiety symptoms. It was fascinating to see the real, tangible data that paralleled what I was feeling and experiencing.
It’s interesting to hear that the average variability in resting heart rate is around 1%. I wonder if this is not true of many women of reproductive age–I find that my heart rate consistently rests around 68 during/after my period, then gradually rises to a high of 74/75 around 10-12 days after ovulation. Searching online seems to indicate that this is normal.