This episode of Longevity by Design is an in-depth discussion on heart rate variability (HRV) and the stress response. Our guest today is Dr. Altini, an HRV expert and data expert. He and Dr. Blander talk through the definition of HRV, how stress impacts the autonomic nervous system, and the many factors that influence HRV. Tune in as Dr. Altini sheds light on how to interpret and improve this highly individualized marker.
About Dr. Marco Altini
Dr. Marco Altini is a scientist and developer whose work is at the intersection of technology, health, and performance. Dr. Altini holds a PhD in Data Science and Master's Degrees in Computer Science Engineering and Human Movement Sciences. Dr. Altini is the founder of HRV4 training—the first validated app to measure heart rate and heart rate variability, giving users actionable insights into their stress and training. Additionally, Dr. Altini is a data scientist at Oura and a guest lecturer. He has over ten years of experience and more than 50 publications modeling physiological data.
What is heart rate variability?
Heart rate variability (HRV) has become an increasingly popular physiomarker that individuals will track to understand their health better. But what does HRV really measure, and how does it reflect our health status?
Heart rate variability is a measure of the variation between heartbeats. While our heartbeats follow a predictable pattern—repeated cycles of a P wave, QRS complex, and T wave—the time intervals between each of these heartbeats can vary. Dr. Altini specifies that our hearts do not beat at a constant rate; rather, there is always some variation, which HRV captures.
HRV and the stress response
The degree of variability between heartbeats is not random; it indirectly measures how our bodies respond to stress.
“When we face a stressor, we have a response from the autonomic nervous system such that our parasympathetic branch—the one in charge of rest and recovery—is suppressed. This suppression impacts heart rhythm; our heartbeats become more constant, and our heart rate becomes elevated,” explains Dr. Altini. Essentially, a low HRV reflects the body being under high stress because the heart rate becomes elevated and heartbeats become more constant. The opposite happens when the body is relaxed or has adapted to the current stress level, resulting in a high HRV.
The difference between heart rate and HRV
So if heart rate and HRV both respond similarly to a stressor, why measure HRV? Dr. Altini discerns that HRV is more sensitive to the stress response, therefore providing valuable information independent of heart rate. "When comparing heart rate and HRV in response to training, we see that the degree of change for HRV is much larger. For example, if the change in heart rate is around 1%, the change in HRV can be between 5-10%," says Dr. Altini.
The sensitivity of the measure HRV is valuable for understanding the impact of seemingly subtle daily stressors. "When it comes to sickness or large alcohol intake, your heart rate and HRV both change dramatically, so HRV is not necessarily something you need to look at since you already know that the change is obvious in heart rate data. For more subtle stressors and day-to-day changes, HRV becomes more sensitive, and that allows us to better understand what is happening in response to all these stressors," he explains. At times, psychological and day-to-day stressors don't impact heart rate but do impact HRV, making HRV a more telling metric for tracking total body stress.
When it comes to interpreting our HRV, the timing of the measurement is critical. To accurately capture our body's response to the most significant stressors in our lives, there are essentially two options: measure HRV continuously throughout the night with a wearable device or measure HRV acutely first thing in the morning. Measuring HRV outside of these time points will capture transitory stressors—such as eating, caffeine, socializing, or movement—which are less reflective of the body's total physiological stress level. These minute-by-minute changes during the day are fleeting and can be a misinterpretation of one's HRV. "Typically, a lower HRV is interpreted negatively. However, HRV acutely decreases during exercise, but we know it comes with positive health benefits. So, it is better to measure in a known condition—during sleep or first thing in the morning—so that we capture the body's response to the totality of stressors instead of an acute stressor at the wrong time," he explains.
Dr. Altini emphasizes that using a device that continuously monitors HRV throughout the night is important. While we are asleep and unconscious, many things are happening at the level of the autonomic nervous system. There can be high variability in HRV during the night, so a single measurement will not represent your physiological stress level but rather the specific sleep cycle you are in. He says that wearables like Oura ring and Whoop are reliable ways to capture HRV because they track your heartbeat throughout the night and present an average score in the morning. He concludes that these sensors have been validated with electrocardiography, making them excellent options for tracking HRV.
HRV and the aging process
One of the defining features of HRV is how highly correlated it is with age. In contrast to heart rate, which can remain stable throughout life, HRV changes dramatically. “If you are fairly active and regularly exercise, your resting heart rate when you are 20 versus 60 can be exactly the same. This is not true for HRV, which declines dramatically as we get older, making age the strongest predictor of HRV at the population level,” explains Dr. Altini. When comparing individuals who regularly exercise to more sedentary people, the between-person variation in HRV is almost non-existent once we reach our sixties and beyond. “In younger people, those who exercise more tend to have a higher HRV. However, because HRV is driven by age, these differences disappear once we reach our sixties.”
Dr. Altini notes that the mechanism behind the age-related decline in HRV has not yet been determined. However, one potential explanation is the decline in our ability to maintain autonomic nervous system (ANS) activity as we age. And as our ANS function declines, HRV declines with it.
Factors that impact HRV
Dr. Alitini explains how biological sex, menstrual cycle, and ethnicity impact HRV.
- Biological sex: The literature is inconsistent about the impact of biological sex on HRV. Some evidence suggests that women have a slightly higher HRV than men, yet other studies contradict this. When looking at the totality of the evidence, HRV is quite similar between males and females.
- Menstrual cycle: Interestingly, the menstrual cycle influences HRV levels. “For a woman who experiences a regular menstrual cycle, you see a strong coupling between different phases of the cycle and HRV. For example, during the second phase of the cycle, there is a typical suppression in HRV. Body temperature also fluctuates during the menstrual cycle, which also impacts HRV.” Dr. Altini says the HRV-menstrual cycle relationship has been well-documented, concluding that the menstrual cycle drives much of the change in resting HRV for those with a regular cycle.
- Ethnicity: Current scientific evidence doesn’t suggest that HRV levels differ by ethnicity, Dr. Altini notes.
How to interpret HRV levels
Dr. Altini cautions that one acute measure of HRV or HR can be misleading. "In the long run, a reduced resting heart rate is great. But acutely—it's typically a bad sign if your heart rate is highly suppressed today compared to yesterday. It means your body is very fatigued, and the same can happen with HRV. If your HRV is abnormally high one day, it probably means that your body is overly parasympathetic. Not because you are super recovered, but because it's working really hard to get you back to your normal state after a hard effort."
So, improving HRV does not necessarily mean having a higher absolute value. There is another aspect to it, which is simply having a more stable HRV. If your HRV fluctuates dramatically—for example, due to being highly stressed or consuming alcohol—your absolute value may be higher, but your HRV is not in an ideal state. "Stress can be good, and sometimes it's essential. The point is not to avoid all stressors, but rather to put yourself in a state where you bounce back very quickly." The goal is to get into a state of stability, often confused with avoiding stress. It's about facing the right stressors, namely the ones you can cope with at a given time.
How to improve HRV levels
Like many other markers, the degree to which someone can improve their HRV depends on their baseline health, lifestyle habits, and fitness level. Dr. Altini explains that improving diet quality, exercising regularly, getting adequate sleep, and implementing stress management techniques are four ways to improve HRV levels. Those with a baseline of suboptimal health practices can achieve a greater degree of change in their HRV. Here are two specific strategies for improving your HRV:
- Exercise: Introducing variety into your exercise regimen—strength training, endurance training, and low-impact exercise can best improve HRV levels.
- Diet quality: Regarding diet quality, Dr. Altini notes that minimizing ultra-processed foods and following a Mediterranean diet are well-studied ways to improve HRV.
Advice on living a healthier longer life
Dr. Altini’s top tip for improving healthspan is to stick to the basics—move throughout the day, eat well, exercise, and of course, try to manage stress.