In a world that increasingly focuses on self-quantification, heart rate variability is a tool that has expanded from the sports performance arena to become a commonplace metric on wearables like the Oura Ring, Apple Watch, and Fitbit.
What is Heart Rate Variability (HRV)?
Heart Rate variability is the variation in the interval between consecutive heartbeats. HRV reflects the balance between the sympathetic nervous system which modulates our response to stress and the parasympathetic nervous system which promotes relaxation, sleep, digestion, and recovery (these are the 2 arms of the autonomic nervous system). A stressful day at work, a poor night of sleep, or an unhealthy meal may overdrive your sympathetic nervous system and push your body out of balance.
Both heart rate and HRV are influenced by autonomic function. Heart rate however tends to have an inverse correlation with HRV in that increased parasympathetic activity increases HRV and lowers heart rate.
Factors That Modulate HRV:
- Genetics: Genetic twin studies show about a 40% contribution from genetics.
- Age- HRV declines with age especially in 2nd and 3rd decades. Below is the average HRV by decade.
- Sleep: Decreased quality of sleep is associated with lower HRV.
- Stress, Anxiety, and Depression: Higher levels of stress (psychological or physical) will decrease HRV.
- Alcohol: may be related to its negative impact on sleep.
- Elevated Insulin levels
- Overall Health: In general, HRV declines with decreased health. Autonomic nervous system dysregulation may contribute to the incidence and maintenance of multiple symptoms of declining health. Physical conditions such as inflammation, chronic pain, and fatigue increase sympathetic nervous system output and create a self perpetuating cycle.
- Exercise: While increased cardiovascular fitness increases HRV, if you regularly do aerobic exercise, take 4 weeks off and you will see a decline in HRV. On the other hand, excessive cardio (longer and more frequent sessions without sufficient recovery) will lower your HRV. Like most things, there is a duality of exercise, you can do too little or too much.
- Meditation, mindfulness, and sleep are also effective tools to increase HRV.
- Biofeedback and breathing exercises can significantly increase HRV
Clinical Correlates of HRV:
Low HRV levels (below 15 or 20) correlate to increased risk of heart disease and death from all causes including cancer.
A study in the American Journal of Epidemiology showed an association of baseline HRV with the onset of cardiovascular disease in the following 3 years.
In a related longer term population study, middle-aged men and women with low HRV (< 15) had an elevated risk of cardiovascular disease and death from all causes, including cancer. Of note, higher HRV levels versus intermediate levels were not clearly beneficial as there was no significant difference in mortality, or cardiovascular disease for people with HRV > 22 versus people with an HRV between 15 and 22.
In a prospective Danish study of middle-aged and elderly men, low HRV was linked to death from all causes, including cancer. The 5-year age-relative risk of total mortality for men with an HRV < 20 compared with men with HRV of 20-39 msec was 2.1 middle-aged men and 1.4 in elderly men.
It is important to recognize that these studies are corrected for multiple variables including age, smoking, and hypertension, but there may be other confounding variables that explain the findings. In other words, HRV may not be causing the clinical outcomes but instead correlates with other variables that are the true drivers.
(Also of note the HRV numbers above reflect rMSSD, an equation for measuring HRV which is used by Oura ring and Apple Watch).
Practical Use Of HRV:
HRV is a useful self-quantification tool that reflects the status of your nervous system and specifically the multiple elements of lifestyle and overall health.
People who have a high HRV may have greater cardiovascular fitness and may be more resilient to stress. More importantly, HRV provides personal feedback about multiple controllable factors in your lifestyle
For Exercise, HRV is a tool that can be used to monitor recovery and guide training intensity. Competitive athletes may use an early morning HRV assessment to guide training intensity. The idea is that overtraining shifts our bodies to a stress state and a lower HRV due to dominance of the sympathetic nervous system. A declining HRV suggests the need for a rest day or low exertion day.
A study of 10 division 1 Swimmers showed a reduction in HRV and perceived wellness with increased training volume and the converse when training was tapered.
For those interested in a peak underneath their own hood, HRV is a novel tool to evaluate the balance of your nervous system, cardiovascular health, and overall wellness.
Dr. Bradford Rabin received his BA from Stanford University with honors in economics. In 1998, he received his MD, from Stanford University, along with the Dean’s award for Outstanding Research for nine neuroscience publications examining the biochemical pathways involved in sleep. Dr. Rabin completed his internal medicine residency at the University of California San Francisco. After his residency, he worked as a clinical instructor in the UCSF Department of Internal Medicine.