A few years ago, I saw an ad for a device that measured heart rate variability (HRV). I didn’t quite get what it was or why it mattered, and figured it was just some company’s way of selling a new gadget. As a longtime health and fitness writer, I had accumulated plenty of gadgets to review throughout the years, most of which ended up sitting in a box or being given away, so I passed on this one. But since then, HRV has gained more attention, including in the academic, research and medical worlds, so I decided to take a closer look at what it is and how it’s connected to health and disease. What I’ve found is fascinating—and not (quite) as complicated as I once thought it was.

What Is HRV and Why Does It Matter?

The human heart beats 24/7, receiving unoxygenated blood from the rest of the body, reoxygenating it with the help of the lungs, and sending it back out to the body. Heart rate goes up and down depending on several factors, including activity. Where HRV comes in is the time or space between beats. And while logic might say that we want our heartbeats to regulate like clockwork, the science says otherwise.

“Your heart doesn’t beat like a metronome,” says Stefanie Broes, PhD, cofounder and CEO of moonbird, a handheld device that helps people build nervous system resilience through guided breathwork and real-time HRV biofeedback. “Even at rest, the time between each heartbeat is constantly changing, and that variation is exactly what HRV measures.”

“Heart rate variability refers to the natural variation in time between consecutive heartbeats,” explains Michael Esco, PhD, FACSM, professor of exercise physiology at the University of Alabama. “It is largely driven by the balance between the parasympathetic—rest-and-recover— and sympathetic—fight-or-flight—branches of the autonomic nervous system.”

“A higher HRV means your heart is more adaptable, which is a sign of a well-functioning autonomic nervous system,” says Broes. “A lower HRV often signals that your body is under stress, not recovering well or running low on resources.”

As a quick anatomy and physiology refresher, the autonomic nervous system (ANS) runs on autopilot and regulates involuntary physiological processes, like heart rate, blood pressure and digestion. The ANS is divided into three separate but connected branches: the sympathetic nervous system (SNS), the parasympathetic nervous system (PNS) and the enteric nervous system (ENS). The SNS speeds things up and is responsible for the fight-or-flight response. The PNS slows things down (think “para” as in a parachute and how it slows the descent), with the vagus nerve being a major regulator of the PNS. The ENS is located in the gut and is responsible for digestion.

Our bodies are constantly trying to maintain homeostasis—a state of dynamic equilibrium— including in our nervous systems. But when the ANS takes over, which is often the case for those who have experienced some level of trauma or chronic stress, our bodies get stuck in fight-or-flight mode, always on high alert.

“HRV matters because it provides a non-invasive window into how the body responds to stress,” says Esco. “When the body is well recovered and ready to perform, HRV tends to be higher, reflecting greater parasympathetic activity. In contrast, when the body is under increased stress, HRV is typically lower, indicating greater sympathetic influence. In these cases, it may signal a need for additional recovery.”

Michele Olson, PhD, FACSM, an academic researcher and instructor, and editorial board member for the American College of Sports Medicine, states that having a higher HRV is desirable. “A high HRV indicates that the parasympathetic nervous system is more ‘toned and in-tune’ with the needs of your organs,” explains Olson. “Fitter, more rested, better nourished and healthier people tend to have higher HRV, indicating that your body and its systems are not ‘tired,’ over-taxed or indicative of over-training, or at risk for serious injury or illness.”

Broes notes that higher HRV is consistently linked to better cardiovascular health, stronger stress resilience, faster recovery and improved emotional regulation. “Your autonomic nervous system becomes more flexible, meaning it can shift between activation and rest more efficiently. For athletes, this translates to better performance and recovery. For everyone else, it often means sleeping better, handling pressure more calmly, and bouncing back faster from difficult periods. It’s essentially a measure of how adaptable your body is, and adaptability is the foundation of long-term health.”

How Is HRV Measured and What Factors Influence It?

According to research, HRV is influenced by several variables, including pathological, physiological, psychological, environmental, lifestyle and genetic factors. Not only has HRV been linked with cardiovascular health, but it’s also associated with emotional and psychological health. And as research continues on HRV, more associations are likely to be revealed.

HRV is most accurately measured with an electrocardiogram (ECG), says Esco, though this is not practical for day-to-day measurements. “Fortunately, many wearable devices—chest straps and some smartphone-based systems using photoplethysmography—have been developed and shown to provide reasonably accurate estimates compared to ECG.”

“Consumer devices like smartwatches, fitness trackers and biofeedback tools use optical sensors—a technology called PPG or photoplethysmography—to detect blood flow changes and calculate HRV from that,” says Broes. “At moonbird, we use PPG for our measurements, and multiple peer-reviewed studies confirm it’s a very accurate method.”

Broes emphasizes that different devices may use different algorithms, so it’s not recommended that you compare your HRV stats from one device to another. Nor should you compare your HRV to someone else’s.

“HRV is highly individual. What is considered 'low' for one person may be normal—and perfectly fine—for another,” says Esco. “Therefore, interpretation should be individual and based on changes relative to a personal baseline rather than comparisons to others.

Esco adds that there is also no one blanket optimal HRV value. “What may be considered as an optimal HRV depends on the individual,” says Esco. “There is no universal ideal value. HRV varies widely based on factors such as age, fitness level, sex and genetics. For example, younger and more aerobically fit individuals tend to exhibit higher HRV, but even within these groups, variability is substantial.”

Esco recommends that rather than targeting a specific number, individuals should focus on establishing a personal baseline or rolling average while monitoring day-to-day and week-to-week trends and noting large deviations, like sustained decreases. “In most cases, consistency and stability over time are just as important as the absolute value.”

Broes agrees. “This is where I’d caution against chasing a number. HRV varies significantly by age, fitness level and genetics. Average HRV tends to decline with age. What matters far more is tracking your own trend over time. If you're consistently sleeping well, managing stress and exercising regularly, your HRV should gradually rise. A sustained downward trend is the real signal to pay attention to.”

If you don’t have a device that calculates HRV, there are a few signs that your HRV may be low. “There aren't always obvious symptoms, which is part of what makes tracking useful,” says Broes. “That said, people with chronically low HRV often report feeling more stressed, fatigued or emotionally flat, without being able to explain exactly why.”

They may also experience insufficient sleep, poor training recovery and a weak immune system, predisposing them to illness, adds Esco.

How Is HRV Improved?

There are several lifestyle factors that influence HRV and can ultimately improve it, starting with regular exercise. “Consistent aerobic and strength training improves HRV over the long term,” says Broes. “Overtraining, however, does the opposite.”

“Improving HRV is less about targeting HRV directly and more about enhancing overall health, physical fitness and physiological resilience,” says Esco.

Esco recommends focusing on the following key strategies, which fall right in line with the ACE 7 Core Drivers of Healthy Living:

• Regular aerobic exercise (one of the most powerful drivers)
• Adequate sleep (both quantity and quality)
• Stress management (e.g., breathing exercises, mindfulness)
• Proper nutrition and hydration
• Limiting excessive alcohol intake
• Appropriate management of training load to avoid chronic overload

“Sleep is probably the single most impactful factor,” says Broes. “Poor sleep tanks your HRV almost immediately, and even moderate alcohol consumption shows up in your HRV the next day. Once you start tracking, this feedback loop becomes very hard to ignore.”

One of Broes’ top recommendations for improving HRV is breathwork, as it is easily accessible anytime. “Slow, paced breathing at around five to six breaths per minute activates the parasympathetic nervous system and directly increases HRV,” says Broes. “This is one of the fastest, most accessible ways to shift your body into a recovery state.”

The Bottom Line

“One of the biggest misconceptions about HRV is that it is a ‘score’ that should be maximized. That is not the goal,” says Esco, who emphasizes that HRV is best viewed as a monitoring tool, not a target. “Its value lies in helping individuals understand how their body responds to training, stress and recovery.”

HRV is also not the only biomarker that should be considered. “While it is a valuable objective marker, it represents only one component of overall recovery and readiness,” says Esco. “Subjective indicators, such as perceived fatigue, sleep quality, soreness and overall well-being, also provide important information. The most effective approach is to combine HRV with these subjective measures.”

Broes agrees. “HRV is powerful precisely because it’s objective. You can feel fine on the surface while your HRV tells a different story, and that's exactly the kind of insight most people are missing,” says Broes. “I'd also encourage people not to become anxious about their numbers. HRV fluctuates naturally day to day, and that's completely normal. The goal is to build awareness of your own patterns and make choices that support your nervous system over time. Think of it as a compass, not a report card.”