There are a number of metrics we can use to get a snapshot of our health and well-being. From blood pressure to heart rate, doctors and researchers are more interested in our physiological data than ever before.
There is one marker for resilience and well-being that researchers have just begun to utilize over the past two decades. It’s called heart rate variability, or HRV. This metric, once measured primarily in athletes and those with abnormal heart rhythms, has since become a key piece of data for individuals wanting insight into the state of their physiology and nervous system.
So what exactly is heart rate variability? How do we measure it? And what can it tell us about our overall health? Let’s break down the intricacies of this emerging physiological measurement.
What Is Heart Rate Variability?
Heart rate variability, or HRV for short, is a measure of the time between each heartbeat. Heart rate by itself is the expression of how many contractions of the heart there are in a given unit of time; however, the rate itself is not constant. There is normal fluctuation of time between heartbeats, in a manner that speeds up and slows down heart rate. Therefore, HRV is a quantifiable measure that assesses these differences.
Regulated by a fundamental part of our nervous system called the autonomic nervous system (ANS), HRV is one of many functions that occurs without us even having to think about it. HRV has been shown to correlate with emotional and physical stress, sleep, and disease making it a common method for assessing the overall physiological state and the rate of adaptation to stressors.
Generally, the higher the HRV the better, as high stress and poorer health outcomes have been associated with low values of HRV.
How Do We Measure Heart Rate Variability?
HRV can be measured by an electrocardiogram (ECG) or photoplethysmography (PPG). By referencing a common point in the ECG or PPG waveform, the time between each heart beat can be recorded in milliseconds (ms). Collecting each beat-to-beat interval in ms allows us to compute HRV, most commonly reported as rMSSD (root mean square of successive differences). The rMSSD method of calculation takes each interval, squares the interval, takes the overall mean, and then the square root of that mean. More complex measures of HRV, including frequency domain analysis, can be used to get further information out of heart rate patterns and the state of one’s nervous system.
What Is a Normal Heart Rate Variability?
HRV has a large individual component that has yet to be understood clinically, and therefore is more often used to assess changes in health over time. HRV can fluctuate day-to-day based on exposure to stress, sleep quality, diet, exercise, and more. This leads to low repeatability, and therefore makes normative data difficult to collect. In general, younger individuals, males, and more active individuals tend to have higher heart rate variability, but the inter-subject variability tends to be too high to suggest proper normative ranges.
Focusing On Trends
As previously mentioned, HRV is difficult to interpret and generally nonspecific using data from a single spot check. However, since it is a dynamic measure that responds to various lifestyle factors, tracking HRV over time allows for non-invasive insight into changes in health status or efficacy of certain interventions. In general, since higher HRV is preferable, a greater ability to manage stress results in an increased HRV. The results of the studies demonstrating the relationship between stress and HRV suggest that interventions aimed at reducing mental and physical stress could increase HRV and minimize day-to-day fluctuations (coefficient of variation, CV%). The increase in HRV itself will not reduce risk and improve health over the long term, but rather, it reflects positive adaptations in an individual’s physiology.
For example, if we’re incorporating exercise or meditation into our daily routine, HRV should steadily increase. A downward trend, on the other hand, may be indicative of overtraining, poor sleep, illness, bad eating habits, increased exposure to stress, or failure to hydrate.
The Biostrap Buzz
Sign up to our email newsletter to receive curated content on the latest news in digital health and health optimization. Plus, special access to Biostrap offers and community updates.
What Factors Influence Heart Rate Variability?
Heart rate variability can be influenced by training, lifestyle, and biological factors.
Training factors that influence HRV include the intensity of a workout, exposure to unfamiliar stimuli, training load, and proper balance between rest days and training days.
Lifestyle factors that influence HRV include diet and nutrition, stress, sleep habits, and alcohol consumption. Leading a healthy lifestyle that focuses on proper diet and physical fitness, while paying attention to mental health, is a valuable means of improving HRV.
Finally, biological factors such as age, gender, genetics, and health conditions can influence HRV as well. As we age, our HRV tends to decline, and men often have higher HRV than women. Genetics and health conditions such as cardiovascular disease are additional factors that may influence our heart’s ability to operate normally.
Should We Focus on Heart Rate Variability?
Measuring heart rate variability is a valuable form of analysis to monitor healthy individuals or to identify those who should seek improvement. The amount of information we get from HRV is making it a popular health data to assess physiological state, overall well-being and stress adaptation. You can track your HRV with clinical reliability with the Biostrap wrist-worn device and keep an eye on your nocturnal HRV as well as weekly, monthly and yearly trends.