What is Heart Rate Variability, and Why Does It Matter?

When you think about your health, how well do you understand heart rate variability (HRV)? Chances are, there are other metrics you are more familiar with. Resting heart rate. Blood Pressure. Cholesterol levels. HRV is another metric doctors can use to assess your heart health. It is the natural variation in time between each heartbeat. Here’s how it is measured, and why it matters:

What is heart rate variability?

Despite the name, heart rate variability, or HRV, does not refer to how fast your heart beats. Instead, it measures "the natural fluctuation in the time intervals between adjacent heartbeats," explains Dr. Shaan Khurshid, a cardiac electrophysiologist in the Mass General Brigham Heart & Vascular Institute.

Even if your heart rate averages 60 beats per minute, your heart is not beating exactly once every second. One beat may occur after 0.9 seconds and the next after 1.1 seconds. Those small variations are normal, and HRV measures them.

This can be useful information to have, in part because HRV reflects how the autonomic nervous system is functioning, the system "that addresses the unconscious 'housekeeping' functions of the body, such as temperature, heart rate, blood pressure, digestion, stress responses and breathing," Khurshid explains.

It does so, in part, because the autonomic nervous system operates through two balancing forces: the sympathetic nervous system, which drives the body’s "fight or flight" response, and the parasympathetic nervous system, often called the "rest and digest" response, which promotes recovery and relaxation. "Since HRV is a result of this balance, it serves as a window into it," says Khurshid. "And when this balance is healthy, HRV tends to be normal."

But a "higher HRV generally means more parasympathetic effect is dominant, suggesting a state of recovery and resilience," explains Dr. Jason Tso, a board-certified cardiologist at Stanford Health Care. Meanwhile, "lower HRV suggests sympathetic dominance, or a state of increased stress."

But it's still probably wise not to read too much into day-to-day changes. "Because HRV is a single readout of a very complex system, it can be very tempting to overread into acute changes and infer a direct cause," Khurshid says. "But in reality, there could be many reasons for an acute change in HRV that frankly we sometimes don’t understand.

There are also technological limitations. Tso notes most consumer wearables rely on optical light-based sensors that are "reasonably accurate at rest but often unreliable with movement," which is why many devices primarily calculate HRV during sleep.

What causes low or high heart rate variability?

There are many factors that can influence HRV. Singh says some of the most common causes of low HRV "include stressors like under fueling, overtraining, poor sleep, sickness, inflammation, medication, illness, alcohol, stress and anxiety." Khurshid adds that fatigue, dehydration and inadequate sleep can also reduce HRV.

Singh says that some of the things that can improve HRV over time include "optimizing sleep and nutrition, managing stress and anxiety and avoiding alcohol." Khurshid says it is also well-known "cardiovascular fitness is associated with higher HRV, likely related to the development of a high-functioning autonomic nervous system."

Other factors can influence both high and low HRV, including travel across time zones, inflammation, menstrual cycle changes, and late-night meals. It's also important to note that HRV naturally declines with age, and some healthy people simply have lower baseline HRV than others.

Indeed, "because everyone’s HRV is different, I always tell my patients to look at their own baseline and trends," advises Singh. In doing so, she says HRV can help people better understand how stress, sleep, nutrition, training and recovery affect them personally, but warns against becoming overly dependent on the number.

"At the end of the day, learn to communicate with your body and listen to your gut," she advises. "HRV is still a calculated measure, but don’t be afraid to ignore it if you feel great and your watch tells you you shouldn’t."

Learn more by reading the full article here.

You don’t need to obsess over your HRV. In fact, it would probably be best if you didn’t. But being aware of what it means and what your normal is can help you work toward your optimal health.