The measure of heart rate variability (HRV) has become increasingly popular and accessible to many with new wearable device technology. HRV is an individualized metric that can help monitor and track the body’s resilience and ability to manage stress. It's a sought-after indicator for those looking to optimize their overall health, performance, and recovery.
Let's dive into everything you need to know about good heart rate variability.
What is heart rate variability?
Heart rate variability is the measurement of the variation in time between heartbeats. The autonomic nervous system (ANS) controls this variation and is responsbile for other critical functions like heart rate, blood pressure, breathing, and digestion. Within the ANS, there are the sympathetic (SNS) and parasympathetic nervous systems (PNS). These systems have competing roles in a dynamic relationship—acting as the flight or fight and the rest and digest responses, respectively. [1,2]
Both of these systems impact heart rate, and therefore heart rate variability. 
- The parasympathetic nervous system sends signals to the body to decrease heart rate during times of rest.
- The sympathetic nervous system sends signals to increase heart rate during times of stress.
When there is an imbalance between these two systems, heart rate variability is impacted. The measure of HRV is indicative of the body’s ability to respond to physiological (internal) and environmental (external) stressors. [2, 3]
What's a good HRV?
What's considered a good HRV will vary from person to person. Multiple formulas are used to calculate this metric—two of the most common are the root mean square of successive differences between normal heartbeats (RMSSD) and standard deviation of normal intervals. Wearable devices use these formulas to compute HRV automatically, and current data show a desirable range may be as broad as 20 to 150 milliseconds—depending on the formula used. [2, 4, 5] However, most of the data on precise ranges of HRV is proprietary information to the companies of the wearable devices that measure it. Public and peer-reviewed research is still evolving, and there’s currently no consensus on what normal values are.
The current scientific literature suggests the best way to monitor and determine a good HRV for you is to track your own trends daily. These trends are a powerful predictor of stress levels, fitness, and recovery. Using a 24-hour measure of HRV is considered the gold standard or most accurate measurement. 
And overall, a higher HRV is more desirable than a lower HRV.
Why is a higher HRV considered better than a lower HRV?
A higher HRV suggests that one system—the SNS or PNS—is not dominating over the other, but rather that the body is flexible and ready to adapt as needed to the input from both.  This adaptability helps the body respond appropriately to stress and return to homeostasis, or relaxation, after an acute stress response (like exercise).
What impacts HRV?
Much like resting heart rate, several factors can impact HRV. These include the following:
Sex: Healthy women have a lower HRV when compared to their healthy male counterparts, especially before the age of 30. The difference between gender gradually decreases until it stabilizes after 50 years of age. [2, 4]
Age: HRV typically declines with the normal aging process, but it's not inevitable.  According to a study of 45 healthy adults by the International Journal of Cardiology, the age-related decline in HRV was not observed in healthy, older adults. While more research is needed, this study indicates that healthy individuals may be able to maintain HRV levels with age. 
Heart rate: A faster heart rate is associated with a lower, or poorer, HRV as there is less time between beats. Conversely, a slower heart rate increases the time between heartbeats, raising HRV. 
Overall health: HRV can be useful in predicting morbidities from conditions that affect mental and physical health. These can impact the SNS, creating an imbalance within the ANS and lowering HRV. This ANS dysfunction is associated with acute and chronic illness and is a risk factor for serious health issues, including cardiovascular disease, stroke, and overall mortality. 
Other factors: Other factors that can impact HRV (and your ANS) that change daily include inadequate sleep, stress, illness, or poor nutrition.
What's the relationship between exercise and HRV?
One of the main reasons HRV tracking has become increasingly popular is to monitor fitness levels and recovery—as physical activity is one of the best ways to improve HRV.
Higher HRV is associated with improved cardiovascular health because as you endurance train, stroke volume increases, meaning you can pump more blood with each heartbeat. Therefore, fewer beats are needed each minute to maintain proper blood circulation. That lowers your heart rate, and a lower heart rate means you have more room for deviation between your lowest and highest heart rate—increasing your HRV. 
How does HRV impact recovery?
A higher HRV is associated with improved recovery from exercise. A meta-analysis and systematic review in Sports Medicine found that measures of both resting and post-exercise heart rate variability increase in response to training, allowing for improvements in athletic performance. The adaptations seen indicate a heightened ability of the PNS to return to the body to homeostasis following an exercise stressor. 
PNS reactivation post-exercise isn’t immediate. So HRV right after concluding a workout will be lower than your true resting HRV. However, PNS reactivation occurs more quickly in aerobically trained individuals, and their body is more primed to begin recovery processes. 
- HRV is a measure of the time between heartbeats and is measured by many wearable fitness devices
- A higher HRV value is associated with positive health and performance outcomes
- A lower HRV indicates less resilience and adaptability of the body to respond to physical stressors (like exercise)
- Both non-modifiable (age and sex) and modifiable factors (sleep, stress, and exercise) can impact HRV
HRV is not yet one of the physiomarkers captured by InsideTracker, however, the science team is actively looking into incorporating it into the platform. While research is underway, InsideTracker offers insights into blood biomarkers related to stress, heart health, inflammation, endurance, and recovery, as well as resting heart rate data from wearable devices—all of which can directly impact HRV.
For example, a high level of hsCRP indicates your body is under stress—whether that’s physical, mental, or emotional. And if that’s the case, InsideTracker may recommend incorporating a yoga practice into your routine. Yoga can help reduce stress levels, and emerging research indicates that it may also improve regulation of the ANS, leading to improved HRV.
While hsCRP levels are typically only captured two- to four-times a year with blood tests—wearable devices can continuously pull data on resting heart rate. Garmin smartwatches, Apple watches, and FitBit devices all currently sync with InsideTracker. The InsideTracker app automatically pulls and evaluates that data daily, and you’ll receive real-time and frequent feedback if a measure deviates from recommendations on how to improve it. Heart rate and heart rate variability are closely tied, so lowering your average resting heart rate supports the goal of optimizing your HRV.
Expert review by Molly Murphy, MPH, RD, LDN
Stevie Lyn Smith, MS, RDN, CSSD, CDNStevie Lyn is a content Strategist and Team Nutritionist at InsideTracker. As a Registered Dietitian and Ironman triathlete, she enjoys combining her passions to help educate others on how to fuel for overall health and performance. When she’s not swimming, biking, or running with her dog, you’ll find her in the kitchen working on a new recipe to improve her biomarkers. Keep up with Stevie on Instagram.