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Heart Rate Variability (HRV): what it is, and why it matters to your health and your sleep

In recent years, the medical profession has also adopted heart rate variability, or HRV. Although it first took hold in a clinical context in 1965, over the years researchers have learned more and more about its causes and effects, and what HRV measurements can tell us about the human body.

The Crescent Team
The Crescent Team

· 4 min read

In response to the stress, challenges, and stimulus we take on during exercise and training, our body is continuously re-adjusting to maintain a state of balance, or homeostasis. In particular, stress causes corresponding reactions in our heart rate, blood pressure, glucose level, hormones, etc. and our autonomic nervous system, like an engine of a car, works to keep everything in balance.

Resting heart rate and peak heart rate have long been used by medical professionals and fitness enthusiasts to measure health and athletic performance. They’re the numbers that most athletes and fitness enthusiasts are familiar with.

In recent years, the medical profession has also adopted heart rate variability, or HRV. Although it first took hold in a clinical context in 1965, over the years researchers have learned more and more about its causes and effects, and what HRV measurements can tell us about the human body.

Simply put, it’s the variance of the time between heartbeats.

Multiple bodily functions control your heart’s behavior. While your heart’s rhythm is controlled by your sinoatrial node, or SA node, your heart rate variability, like many other automatic processes, is controlled by your autonomic nervous system, or ANS.

The autonomic nervous system consists of two branches:

  • The sympathetic nervous system (often called “fight or flight”) reflects responses to things like stress and exercise, and increases your heart rate and blood pressure.
  • The parasympathetic nervous system (often referred to as “rest and digest”) handles inputs from internal organs, like digestion or your fingernails and hair growing. It causes a decrease in heart rate.

Your HRV is an indicator of your body’s ability to manage these conflicting signals. This makes measuring HRV an effective way to capture how our body is doing while trying to maintain a state of balance in response to different stressors (training, lifestyle, etc.).

In general, a higher HRV indicates better physical health. It means that your body is better able to react to stress, and once the stress has been relieved, it can quickly bounce back. Your HRV is an indicator of how well your ANS and your heart are working together.

A low HRV is generally bad. It can mean that your body is dealing with too much stress, and your sympathetic nervous system is working overtime.

Some of the many factors that affect HRV

These causes of stress can be chronic (such as obesity, alcohol abuse, or a long-term illness), and studies have shown a correlation between low HRV and incidence of coronary disease.

Temporary causes of stress can also affect HRV in the short term. Job stress, over-exercising, or a lack of quality sleep can affect your HRV. If you’re otherwise feeling fine but your HRV drops over a short period of time, it can be an indicator of something wrong, such as an undetected illness.

HRV and Athletic Training

When you catch a cold or the flu, it may be several days before you begin to feel the obvious symptoms. A drop in your HRV, when you’re otherwise feeling fine, can indicate an illness.

Highly fit athletes are often less susceptible to the symptoms and effects of an illness. That can be a problem, because over-exercising during an illness can cause long-term performance issues like chronic inflammation and a weakened immune system.

Training adaptation with HRV

This is where HRV monitoring comes in. Athletic trainers review daily HRV measurements and can reduce training loads when they see lower HRV, to allow athletes recover more quickly and return to their normal training schedules. An improved HRV is a more accurate indicator of your body’s recovery from a cold or flu than whether or not you’re still feeling symptoms.

How Crescent Uses HRV Data

Use Crescent's mobile app for more in-depth analysis, and actionable insights into what's impacting your HRV and recovery

Your HRV is tracked by your sleep tracker (such as an Oura Ring, Whoop, Apple Watch, or virtually any other Apple HealthKit or Google Fit compatible device) and that data is processed by the Crescent mobile app. Our sleep coaches review and interpret your HRV trends to provide you with guidance on how to best respond and move forward. Poor sleep health has a key effect on your HRV, and you’ll find that as you work with your sleep coach, your HRV will improve.

What's the Right HRV?

Everybody’s HRV is different. It can vary due to so many factors, like age, gender, and overall health. It also fluctuates throughout the day, and between days. And, for most people, it gradually goes down as they age.

While researchers have compiled statistical data on typical HRV ranges by age and gender, the more important metric is how your individual HRV changes over time.

The study of HRV is ongoing. This article has presented a broad overview of what it means, and how it’s measured. As always, if you have questions or concerns about your health, speak to your doctor.


  1. AHA Journals: Heart Rate Variability: Standards of Measurement, Physiological Interpretation, and Clinical Use
  2. Harvard Health Publishing: Heart rate variability: A new way to track well being
  3. National Institutes of Health: Modeling the association between HR variability and illness in elite swimmers
  4. AHA Journals: Low Heart Rate Variability in a 2-Minute Rhythm Strip Predicts Risk of Coronary Heart Disease and Mortality From Several Causes

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