Sharing exerts from original articles on HRV, published on Harvard University Health Blog (2017) and Oura website 2020.
What is HRV?
As popular as the metaphor may be, a healthy heart doesn’t beat as regularly as a metronome—it actually changes its rhythm with each beat. This constant variation in milliseconds between your heartbeats is known as your heart rate variability (HRV).
HRV is simply a measure of the variation in time between each heartbeat. This variation is controlled by a primitive part of the nervous system called the autonomic nervous system (ANS). It works regardless of our desire and regulates, among other things, our heart rate, blood pressure, breathing, and digestion. The ANS is subdivided into two large components, the sympathetic and the parasympathetic nervous system, also known as the fight-or-flight mechanism and the relaxation response.
The brain is constantly processing information in a region called the hypothalamus. The hypothalamus, through the ANS, sends signals to the rest of the body either to stimulate or to relax different functions. It responds not only to a poor night of sleep, or that sour interaction with your boss, but also to the exciting news that you got engaged, or to that delicious healthy meal you had for lunch. Our body handles all kinds of stimuli and life goes on. However, if we have persistent instigators such as stress, poor sleep, unhealthy diet, dysfunctional relationships, isolation or solitude, and lack of exercise, this balance may be disrupted, and your fight-or-flight response can shift into overdrive.
Some situations result in an increase in variation (high HRV), while others cause the intervals between beats to stay more constant (low HRV).
You may be unaware of these subtle variations, but they reflect your heart’s ability to respond to different situations. HRV can react to stress and/or illness before resting heart rate (RHR), which makes it one of your body’s most powerful signals—providing useful insights into your stress levels, recovery status, and general well-being.
Heart Rate Variability Reflects Your Nervous System
HRV is linked to your autonomic nervous system (ANS) and the balance between the parasympathetic (rest-and-digest) and sympathetic (fight-or-flight) branches. By balancing the two forces, the ANS helps you respond to daily stressors and regulate some of your body’s most important systems, including heart rate, respiration, and digestion.
HRV is the most effective way of estimating your ANS balance, as it directly impacts your heart’s activity.
Independently, your heart beats to its own rhythm thanks to a built-in pacemaker called the sinoatrial (SA) node. Your SA keeps your heart firing at around 100 beats per minute.
However, your ANS greatly influences how your heart beats. All the cells within your heart’s pacemaker have direct phone lines from both sides of your ANS. They give input, beat to beat, on a cellular level to your heart:
- Your rest-and-digest tells your heart to slow down, making room for variability between beats (higher HRV)
- Your fight-or-flight system tells your heart to speed up, limiting space for variability (lower HRV)
These two systems can be silent, active, or shouting over each other at any given time. That’s a lot of conversation, and it leads to a lot of variation.
As a rule of thumb:
- High HRV is associated with rest-and-digest, general fitness, and good recovery
- Low HRV is associated with fight-or-flight, stress, illness, or overtraining
Heart Rate Variability is Unique to You
“High” and “low” HRV is relative for each person. HRV is a highly sensitive metric, which responds uniquely for everyone.
Some individuals have steady HRV scores, while others fluctuate greatly. HRV is an evolving tool, which means, at every HRV level, your personal scores and body status observations are especially important.
A normal HRV for adults can range anywhere from below 20 to over 200 milliseconds. The best way to determine your normal level is to use a wearable that measures your HRV in a controlled setting, like sleep, and establishes a baseline over a few weeks.
Each person’s HRV is unique, so compare your HRV to your own averages and avoid comparisons to others. It’s normal to see daily and seasonal fluctuations in your HRV.
Keep in mind that there are many factors that impact your ANS and, therefore, your HRV:
Heart Rate Variability Has Short and Long-Term Trends
Because HRV is so responsive, there are multiple touchpoints where it can provide insights. Helpful metrics to track are:
Each metric provides different insights.
How your nightly average HRV compares to your baseline, and whether it trends up or down, can help you know how to approach your day (e.g., prioritizing rest or facing a challenge).
Comparing your daytime check-ins to your general baseline can give you insight into how certain activities, like meditation or exercise, affect your system.
How your HRV trends over a few weeks, your HRV balance, can help you identify if you’re successfully rebounding from taxing days (e.g., training, illness, stress).
How your HRV changes over months can reveal how major lifestyle changes (e.g., a new job, becoming a parent) are impacting your health.
Heart Rate Variability Has Natural Highs and Lows
Some explanations of HRV oversimplify it by asserting that a high number is always good and a low one is always bad. It’s far more complex.
While high HRV is generally positive, there are situations where low HRV is necessary and even desirable. For example, during strenuous exercise, low HRV is a reflection of your fight-or-flight system appropriately dominating to get your heart rate up for activity. Your HRV will rebound afterwards, as your rest-and-digest system takes over to help you recover.
These fluctuations occur throughout the day, as different daily stressors continuously challenge your system. Simply being excited, or moving from the couch to the bathroom, can change your HRV at any given moment.
By measuring your HRV during the day, you’re capturing your body’s response to these fleeting changes. If you want to understand your chronic physiological state, the best time to measure HRV is while you sleep.
At night, your body is in a consistent state, without any confounding variables (e.g., food, social interactions, changing environment). Some wearables take the apples to apples approach when it comes to your measurements. They fail to caveat that daytime measurements might mask your underlying ANS balance.
Which Patterns Matter?
Monitoring your HRV can help you reach peak productivity, manage stress, and fine-tune your training regimen.
A single nightly HRV reading can show you:
- A higher HRV score that reflects a rest day, cool bedroom, or “mindful” low/moderate intensity activities such as hiking or yoga
- A lower HRV score that results from dehydration, alcohol, late meal or exercise, illness, a high-intensity workout, acute stress, or a hot bedroom
You may notice that your HRV varies greatly from day to day. If you’re looking for patterns, what matters is your HRV trend. Are you moving up or down over time? Your HRV balance can tell you.
Heart rate variability and myocardial infarction: systematic literature review and metanalysis. European Review for Medical and Pharmacological Sciences, July-August 2009.
Reduced heart rate variability and mortality risk in an elderly cohort. The Framingham Heart Study. Circulation, August 1994.