Step counting and calorie tracking, step aside—there’s a new metric on the block that everyone’s obsessing over: heart rate variability (HRV). But despite its rising popularity, many of us might be misinterpreting what HRV truly represents and how it should be used.
According to Marco Altini, PhD, data scientist and HRV expert, many of us have been looking at heart rate variability wrong in many ways. “While HRV is often over-simplified, with a focus on increasing it over time, I do not necessarily believe this is the best use for this metric.”
What is Heart Rate Variability (HRV)?
Our hearts do not beat with the monotony of the hands of a wall clock. When we say that a person has a heart rate of 60 beats per minute, it doesn’t mean their heart beats exactly every second of the minute, but rather that there can be a variety of time intervals between each beat, for example, 1.2 seconds or 0.8 seconds. This variability in the timing of heartbeats is what HRV refers to.
Altini explains that this variability matters because it is modulated by the activity of the autonomic nervous system in response to stress when measured according to best practices.
“In particular, heart rhythm at rest is regulated by the parasympathetic branch of the autonomic nervous system, the one in charge of rest and recovery. Hence, measuring HRV at rest is an effective and non-invasive way to capture our body’s response to stress (training, lifestyle, etc.).”
In other words, it can indicate how well your body copes with stress and recovers from exercise, illness, and other stressors.
How HRV Can Be Misinterpreted
One common misconception is the overemphasis on increasing HRV. Altini notes that the absolute value of our HRV is influenced by many factors including genetics, age, gender, lifestyle, and behaviors. When people first start tracking their HRV, many fall into the trap of comparing their numbers with others, turning it into a competition of who has the highest HRV.
This approach can be misleading because individual HRV readings are highly personal and not directly comparable. If the basics are covered—regular exercise, adequate sleep, and a balanced diet—chasing a higher HRV might not only be unrealistic but also unnecessary. Instead, “a stable HRV, more than a higher HRV, is what we should probably aim for, under most circumstances,” Altini recommends.
What’s a ‘Normal’ HRV
Understanding what “normal” HRV means for you is the first step. Unlike other metrics, such as blood pressure or blood sugar, it does not have a standardized “normal” range that is determined at the population level.
“We should collect data for several weeks, and use a tool, like an app, that can determine our normal range,” Altini explains, adding that normal range then becomes your individual frame of reference, your baseline. Only then can you accurately determine if a reading is unusually high or low.
What Happens When Your Heart Rate Jumps or Falls?
Once we have our normal range or baseline, we can compare daily or weekly readings against it to determine if our current HRV is abnormally low or high. “A particularly low value normally is representative of high stress, and under these circumstances, it might be ideal to implement changes,” Altini underscores, adding that we can try to reduce training stress, by reducing intensity or prioritizing sleep and recovery.
On the flip side, while a higher HRV is often considered a good sign, this is true only in the long run, when things are changing very slowly and gradually. A positive behavioral change such as giving up alcohol, starting a tailored nutrition program, or picking up exercise after a long sedentary period may all show up in your readings as long-term positive adaptations.
On the other hand, an acutely abnormally high score might indicate that the body is overly parasympathetic, for example, as an attempt to recover from a strong stressor. This is a good reason why we need to be careful with oversimplifications and might simply be better off looking at maintaining our HRV within our normal range.
To monitor how much your HRV fluctuates over time, take a peek at the Coefficient of Variation, or CV (most wearables and apps will show this). Altini explains that this percentage is relative to each person, and it can tell you a lot about your HRV stability.
If your CV is low, your HRV doesn’t swing wildly, which may suggest that you bounce back from stress well. On the other hand, if your CV is high, there may be a lot of ups and downs, indicating that stress might have a bigger impact on your nervous system. Tracking these trends can help you understand how well your body responds and adapts to stress and when you might need to make some changes to improve your resilience.
HRV Relationship To Stress
Altini is cautious about the trend of continuous HRV monitoring and stress assessments throughout the day offered by wearables as it can lead to data misinterpretation. “HRV is not a measurement of stress and is not even a measurement of the stress response when measured continuously, a common misconception somewhat promoted by today’s wearable trackers,” he underscores, adding that HRV is sensitive to minor, inconsequential activities that may cause spikes not related to your stress response.
For example, drinking water changes blood volume in ways that can increase HRV for up to an hour, Altini explained, adding that “none of these changes are actual changes in our stress response, but simply artifacts, as HRV is modulated by a myriad factors, and not only parasympathetic activity.”
Instead, he recommends measuring it by using standardized protocols—like first thing in the morning or during undisturbed sleep. These are more reliable snapshots of your autonomic nervous system without the noise of daily activities.
Take Your ‘Readiness’ Scores With a Grain of Salt
If you’ve ever felt thrown off by the readiness score your wearable showed you in the morning, you’re not alone. A study that looked at how people who exercise regularly use their readiness scores to determine training intensity found that while most reported it to be valuable to connect the dots between behaviors and lifestyle choices and their sleep and recovery, it’s also important to consider subjective feelings of readiness rather than merely relying on the numerical score.
These scores are proprietary algorithms in which different metrics captured weigh in to a varying degree and are unique to each wearable. “They fail to capture much of the picture and give the false impression that, by aggregating data, they are providing us with a comprehensive overview of our state,” Altini points out. “They tend to dilute the insight and make assumptions on our response that might not be applicable to us. For example, muscle soreness is not reflected in the readings, but is typically the main limitation in many sports in terms of training periodization.
Should We Look at Another Metric Before HRV?
If HRV isn’t the golden ticket to gauging stress resilience, what should we be measuring instead? “We often overcomplicate things, and to anyone interested in tracking their stress response and resting physiology, I would maybe recommend starting with a simpler and more reliable marker, such as resting heart rate,” Altini suggests.
Resting heart rate is similarly indicative of our stress response, with a higher value typically associated with increased stress. It also tracks changes in cardiorespiratory fitness better than HRV does. Additionally, it is much more complicated to measure HRV accurately, than it is to measure resting heart rate. Altini points out that the noise in the signal acquired, especially with optical sensing typical of wearables, is much larger on HRV than it is on heart rate.
The Future of Heart Rate Variablity
HRV research still has limitations and room for improvement, particularly in understanding how it can be used as a practical tool for improving health and performance. Altini explains that the challenge is not just improving HRV, but investigating what the implications are of changes in HRV.
Sometimes the assumption, he says, is that by changing the metric, we also change the outcome—whether it’s longevity, health span, or performance. “I do not think we currently have evidence to show that improving HRV leads to improved outcomes, and therefore, I am often trying to shift the conversation toward using HRV as a feedback loop to make meaningful adjustments in our behavior and/or environment, so that what we improve in the long term is not HRV itself, but health or performance. Understanding better some of these links I believe will be key to making better use of the data and of available interventions in the future.”
Is it Worth Tracking HRV?
The short answer is yes. But the context and data quality matter. HRV is a more sensitive non-invasive marker of stress than heart rate due to the nature of how parasympathetic activity modulates heart rhythm.
However, instead of tracking it nonstop, HRV can be more valuable as a feedback loop in relation to acute stressors. This then might allow you to implement meaningful changes in a more timely manner. All that as long as the data captured is high-quality.
“My recommendation is to look at the actual data, not at made-up estimates built using HRV and other signals so that we can better understand our response to stressors, and possibly make meaningful adjustments.”
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