By Nermeen Nabil Khear 
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Heart rate variability (HRV) and resting heart rate (RHR) are the metrics that most wearables’ “recovery” numbers are based on. I wore five devices to bed for two weeks to see how their readings compared. Those devices were an Apple Watch Series 10, a Fitbit Charge 6, a Garmin Forerunner 265S, an Oura Ring 4, and a Whoop 4.0.
Why these five? Well, they’re the major brands that people tend to gravitate toward when they want to track sleep, and for each I chose the best (in my opinion) of each company’s current offerings. I had most of them on hand due to testing them for recent or upcoming reviews. The Apple Watch Series 10 is the one our Associate Tech Editor Michelle Ehrhardt crowned the best Apple Watch for most people. The Charge 6 is Fitbit’s standout no-nonsense fitness tracker (sorry to Fitbit smartwatch lovers, but there are better smartwatches out there.) The fourth-generation Oura ring is the company’s newest and best, and Whoop’s 4.0 strap is the current hardware for its subscription-based recovery and activity tracking service (which beat Oura in our head-to-head comparison). Garmin makes a multitude of watches that can track your sleep, but the one that I tested here is my personal device, a Forerunner 265S—which is, in my opinion, one of the best running watches out there.
As I’ve discussed before, the numbers we get from our smartwatches fall into a few different categories. Many are scores or icons that we can’t really verify with other devices—what does it mean to have a “sleep score” of 87, anyway? Others are measurements, and we can compare those from device to device, since they should all be measuring the same thing. Different devices may use different sensors to pick up the data and different algorithms to process and display it, but we’d expect to see similar numbers from all of the devices tested. Our HRV and resting heart rate numbers fall into this latter category, so let’s see how well the devices match.
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What is HRV, anyway?
HRV, or heart rate variability, is a measure of how steady your heartbeat is. Not how fast or how slow, but how different of a time frame passes between beats. The differences are only measurable with precise equipment, but here’s the basic idea: If your heart goes “beat…beat..beat………..beat….beat..beat…….beat,” you have a high HRV (high variability), and that’s a good thing. On the other hand, if your heart goes “beat….beat….beat…beat…..beat,” that’s a low HRV. We tend to have a low HRV when we’re stressed or fatigued, and a high HRV when we’re either not stressed, or recovering well from our stressors.
This may seem counterintuitive, because most of us think of an ideal heartbeat as being steady and regular, but subtle variations from one beat to another are healthy and normal. Our heart takes its orders on how fast to beat from two different parts of our nervous system (sympathetic and parasympathetic). It’s thought that a high HRV indicates that the two systems are both active—balancing each other out, in a sense.
Your HRV will change from day to day, and that’s where these devices come in. You’ll often see a change in your HRV numbers right after a hard workout or stressful work day, giving you an early heads up about what your body is dealing with. This information, used wisely, can help to guide your choices about how to manage your workout schedule, your stress, and your health-related habits.
What counts as a “good” HRV depends on your own history; it’s not worth comparing yourself to others. (I don’t mean that in a feel-good kind of way, but literally, scientifically: your HRV only makes sense when compared to itself.) Whoop reports that the middle 50% of 20-year-olds have an HRV between about 60 and 105, with numbers declining as we get older. If you’re 60, according to that data set, you’re likely to have HRV numbers somewhere between 30 and 50.
How does HRV differ from resting heart rate (RHR)?
Your resting heart rate is also a number that reflects stress on your body, including fatigue and illness. Unlike HRV, where higher is “better,” a higher resting heart rate means you’re more stressed, and a lower one is a sign that you’re well recovered.
Resting heart rate can also change over time as you become more fit. Athletes tend to have lower RHR numbers, and people who take up an exercise habit often find that their RHR declines a bit over time. (That said, RHR isn’t a direct measure of cardio fitness; there seems to be a significant genetic component as well. Take me as an example: my RHR is always pretty low, even when my cardio fitness is crap. If I’m doing a lot of endurance training, it will drop by two to three points, but no more.)
Most healthy adults have a resting heart rate of between 55 and 85 beats per minute (some sources give 60 to 100 as the typical range). It’s important to note that these numbers usually assume that you’re sitting quietly in a doctor’s office. When you’re asleep in your own bed, your heart rate can dip a bit lower. So it’s normal to see lower numbers on a wearable that records all night than you would see when you go to the doctor.
How to track your HRV and resting heart rate with wearables
Gone are the days when you may have charged your fitness tracker at night; now, fitness-focused wearables are expected to be worn in your sleep to track these nighttime metrics.
During the night, your device monitors your heartbeat. It may sample and average different readings, and each device measures and calculates its numbers slightly differently. (That’s why I didn’t expect much agreement between the devices, but more on that in a minute.) When you wake up, you might be presented with a “readiness” or “recovery” score, but I stand by my position that HRV and RHR are the only numbers really worth paying attention to (aside from time in bed, if you want to monitor your sleep).
This data on your HRV and RHR will be presented to you in the device’s companion app or, in the case of devices with a screen, on the device itself. Often the app or device will tell you how your numbers compare to your usual. What you do with that information is up to you.
How I gathered my data
For this experiment, I wore my five devices to bed every night. That meant:
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The Oura ring on my finger (right hand, middle finger)
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The Whoop band on my left bicep
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The Apple Watch and Fitbit on my left wrist
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The Garmin on my right wrist
A pro tip for device testers: when you’re wearing two watches on the same wrist, turn one toward the underside of your wrist. That way, the two watch bodies won’t clack against each other. Also, make sure the buttons are aligned so they aren’t going to bump against their neighbors in the middle of the night.
I entered each day’s resting heart rate and HRV from each device into a spreadsheet. Garmin reports your seven-day rolling average as your HRV “number,” but I used the nightly numbers instead, the better to match the other devices.
When it comes to using this data, I already have a sense of how the numbers compare to my experience. If my HRV is high and my RHR is low, I pretty much always feel good and am ready to take on whatever the day throws at me. If my HRV is low and my RHR is high, I’m either stressed, sick, or maybe didn’t sleep enough; I may or may not do an easier workout that day, but I’ll definitely pay more attention to taking care of myself and getting plenty of sleep going forward. If my RHR and HRV are both high, that tends to mean I’m dealing with a lot of stress or fatigue but am handling it well. For this experiment, I didn’t bother tracking data on how I felt; I already know that the Oura data is good at matching how I feel, so the question was whether the other devices tracked the same trends or not.
The results of my test
It was fascinating to watch the results take shape as I added more data points to the spreadsheet. The raw numbers were often pretty different: my resting heart rate on a given night might be 65 or 86 depending on which device I’m reading from. But as I logged weeks’ worth of data, the trend lines all told pretty much the same story:
Credit: Beth Skwarecki
For HRV, Oura almost always gives me the highest (“best” looking) readings. Whoop tends to stick pretty close, with Fitbit and Garmin not far behind. The Apple Watch, on the other hand, rides significantly lower on the chart than the other devices, and seems to disagree with their trend sometimes.
For resting heart rate, things are a bit more consistent. Oura usually gives the lowest readings, flattering me again, with the other devices slightly above, and Fitbit giving the highest of the bunch. This is roughly a 10-beat difference for many of the days I charted: for example, 53 from Fitbit and 43 from Oura. It’s hard to say which is right, since I’m not hiring a healthcare professional to stand by and take my pulse throughout the night.
In general, the lines tend to all go up together, and all down together. (Mostly…looking at you, Apple Watch.) I’m happy to see that the devices aren’t reporting drastically different readings that look like random numbers; they do all seem to be measuring the same underlying phenomenon even if they don’t all agree on the exact number to label it with.
Personally, when I want to look at my HRV or RHR, I tend to go with the device I’m most familiar with—in this case, the Oura ring. I’ve been wearing it the longest (through three generations of the product) and so I have a sense of whether a reading of 50 is high or low for me. (It’s high for Oura, even though it might be a low number if I saw it on a different device.)
The most important lesson I take from this, myself, is that not only is it useless to compare HRV from person to person, it’s also not helpful to compare it from device to device. Just like the weight labels on the machines at the gym, the numbers give you a way to compare your progress or trends with the same equipment. Switch devices, and all bets are off. So pick a device, stick with it, and you’ll probably get numbers worth paying attention to. What you do with them, of course, is another matter.
This articles is written by : Nermeen Nabil Khear Abdelmalak
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