What your heart rate variability really says about you

In an age where nearly everyone wants to measure and optimize their health, wearable devices have become a part of everyday life. Many people now use fitness trackers to count their steps, smartwatches to monitor resting heart rates, sleep trackers to evaluate recovery and even continuous glucose monitors to follow their blood sugar trends in real time. One reason for this growing interest is the desire for insight and predictability.

"I think trends over the years, particularly with the advancements in wearables, have focused on being able to predict outcomes and recovery for the day ahead," explains Dr. Tamanna Singh, director of the sports cardiology center at Cleveland Clinic. "People like to be able to anticipate things because most people don't like surprises."

Indeed, that desire for insight is one reason heart rate variability has become increasingly popular in fitness and wellness circles. Here’s what it is, how it works and what can cause it to rise or fall. As with anything else, consult a licensed medical provider before embarking on your wellness journey.

What is heart rate variability? What is a good HRV?

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."

How can knowing your heart rate variability help you? 

At the same time, HRV is highly individual. "A good" HRV for one person may look very different in another because "methods of measurement vary and norms vary with age, sex and fitness levels," cautions Tso.

But that doesn't mean the data is unimportant or meaningless, especially when HRV is considered alongside other health information such as sleep quality, resting heart rate, activity levels, recovery patterns or stress metrics. Researchers and cardiologists have studied HRV for decades, notes Khurshid, particularly as a marker of autonomic nervous system function and cardiovascular health.

What has changed is public access to the data as today, people can measure HRV through smartwatches, fitness bands, chest straps and health apps.

For some people, those scores and trends can encourage healthier habits. For example, athletes sometimes use HRV to balance hard workouts with recovery, while others may notice how sleep, alcohol, stress or hydration are affecting their readings.

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 that 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 "that 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."