Buzz
Heart rate variability (HRV) is a key metric for assessing the level of stress on your body. Many wearable devices track your HRV overnight and provide the results the following morning, along with your resting heart rate, which is another valuable indicator. But which device does this the best? I tested three different devices for three weeks to compare their performance, and the results were quite surprising.
The devices I evaluated included the Oura ring (gen 3), the Whoop 4.0 band, and the Garmin Forerunner 265 smartwatch. (Expect a full review of the Forerunner soon. Spoiler: I’m a fan.)
What exactly is HRV?
HRV, or heart rate variability, gauges the consistency of your heartbeat. It doesn’t measure speed, but rather how irregular the intervals between beats are. While precise equipment is needed to measure it, the concept is simple: if your heart follows a rhythm like “beat…beat…beat………..beat…beat..beat…….beat,” your HRV is high, which is a positive sign. In contrast, if it goes “beat….beat….beat...beat…..beat,” your HRV is low. Low HRV often occurs when we’re stressed or fatigued, while high HRV indicates relaxation or recovery from stress.
This may seem unexpected because we often view an ideal heartbeat as steady and consistent. However, minor variations between beats are normal and healthy. Our heart rate is controlled by two parts of the nervous system (sympathetic and parasympathetic). A high HRV suggests that both systems are actively balancing each other out.
Your HRV can vary daily, and that’s where these devices come in. After a tough workout or a stressful day at work, you might notice a shift in your HRV, offering a sneak peek into how your body is coping. When used properly, this data can help inform your workout routine and health-related habits.
What qualifies as a “good” HRV is unique to you and your personal baseline; don’t compare yourself to others. (Not in a motivational sense, but from a scientific perspective: your HRV only makes sense when viewed in the context of your own measurements.) According to Whoop, the middle 50% of 20-year-olds have an HRV between 60 and 105, with a decline as we age. If you’re 60, for example, your HRV will likely fall between 30 and 50.
What’s the difference between HRV and resting heart rate (RHR)?
Your resting heart rate is also a key indicator of the stress your body experiences, including fatigue and illness. Unlike HRV, a higher resting heart rate suggests higher stress, while a lower rate typically signals that you’re recovering well.
Resting heart rate can shift as you become fitter. Athletes generally have lower RHR values, and those who develop an exercise routine often notice a gradual decline in their RHR over time. That said, RHR isn’t a precise measure of cardiovascular fitness; genetics play a big role as well. For example, my RHR stays fairly low even when my cardio fitness is poor. If I engage in endurance training, it drops by two to three points, but no more.
Most healthy adults have a resting heart rate ranging from 55 to 85 beats per minute (some sources list 60 to 100 as the typical range). It’s worth noting that these figures are based on the assumption you’re seated quietly in a doctor’s office. When you're asleep in bed, your heart rate can drop a bit lower. So, it’s normal to see lower readings on wearables that track all night.
How to track your HRV and resting heart rate with wearables
Gone are the days when you’d charge your fitness tracker overnight; now, wearables designed for fitness are meant to be worn while you sleep, tracking these overnight metrics.
Throughout the night, your device tracks your heartbeat. It may take multiple readings and average them, with each device calculating its results in its own way. (That’s why I didn’t expect much consistency across the devices, but more on that later.) When you wake up, you may see a “readiness” or “recovery” score, but I firmly believe that HRV and RHR are the key numbers to focus on (along with time in bed, if you’re tracking sleep).
The data is presented via the device’s companion app or directly on the device’s screen if it has one. The app or device may also show you how your numbers compare to your usual readings. What you choose to do with that information is entirely up to you.
How I collected my data
For this experiment, I wore all three devices to bed each night.
I own the Oura ring and wear it every night, whether I’m writing about it or not. I’m very familiar with the numbers it gives me. I wore it on my left ring finger.
The Forerunner 265 went on my left wrist. I’m currently testing it out and plan to review all of its features soon, so it made sense to wear it to bed and compare its numbers with the ones I usually get from the Oura.
Lastly, I included a Whoop band in the experiment. I don’t typically wear this (unless I’m writing about it), and for this trial, I only wore it at night, not during the day.
I logged my nightly resting heart rate and HRV from each of my devices into a spreadsheet. Whoop and Oura each provide a single HRV number per night, while Garmin gives two metrics: an 'overnight average' and a 'highest 5-minute average.' I tracked both for Garmin. I wore the Oura and Garmin for about a week before deciding to add the Whoop to the mix. Over the course of a month, I wore at least two devices at a time, collecting data for 24 days across all three.
Regarding how I interpret this data, I already have an understanding of how the numbers correlate with my physical state. When my HRV is high and my RHR is low, I tend to feel great and ready to face whatever challenges the day brings. If my HRV is low and RHR is high, I’m usually stressed, sick, or perhaps not rested enough. On such days, I might choose an easier workout but prioritize self-care and adequate sleep moving forward. When both my RHR and HRV are high, it typically indicates that I'm handling significant stress or fatigue well. I didn’t track how I felt in this experiment, as I already know the Oura data aligns well with my experience, so the focus was on whether the other devices followed the same trends.
Results
It was intriguing to observe how the results unfolded as more data points were added to the spreadsheet. While the raw numbers varied significantly—my resting heart rate on any given night might be recorded as 45, 47, or 50 depending on the device—the trend lines consistently conveyed the same overall story as weeks of data accumulated.
In the resting heart rate chart, it's evident that despite differing raw numbers—with Oura showing the lowest and Whoop the highest—the devices moved in sync, rising and falling together.
If I were using just one device and decided to switch—say, replacing my Oura with a Whoop—it would take some time to adjust to the different numbers. A resting heart rate of 50 on Oura feels quite high (anything above 50 usually makes me wonder what went wrong), while on Whoop, it seems more moderate (‘I guess I’m slightly more stressed than usual’). But once you get used to the numbers, any of these devices would be reliable for tracking resting heart rate and alerting you when it’s unusually high or low.
A similar trend appears in my HRV data, although the differences between devices are more noticeable. Whoop and Oura are nearly identical in their readings—it's even hard to distinguish the light gray line from the blue one on the chart, as the former almost blends into the latter.
Garmin shows a different pattern, with its average consistently slightly lower than that of Whoop and Oura, though its 'highest 5-minute average' frequently exceeds theirs. I’d trust any of these devices, but comparing them directly is not ideal. If I alternated between Oura and Garmin on different nights, I could get a skewed impression. However, sticking with one device provides dependable numbers.
The key takeaway
All three devices—the Oura Ring (gen 3), Whoop Band 4.0, and Garmin Forerunner 265—provided consistently reliable readings for both resting heart rate and heart rate variability. While the numbers varied across devices, the overall trend was the same. I was impressed by how closely the trends aligned. From this data, I wouldn’t pick one device over another for its accuracy; all three are equally suited for the task.
