HRV in Plain English

OCTOBER 18, 2024

By Ryan Yost

You’ve probably heard the term HRV or Heart Rate Variability a lot in the sleep world over the past couple of years. The most popular wearable devices on the market right now have built multi-million dollar businesses around it. But what exactly is HRV, and why is it relevant to sleep?

HRV stands for Heart Rate Variability, which refers to how inconsistent your heart rate is.

• High HRV = Good
• Low HRV = Bad

Why?

• High HRV means your heart rate is highly variable. Sometimes it's slow, and other times it's fast. This shows your body’s ability to switch between relaxing (parasympathetic) and being active (sympathetic). Essentially your body can use its resources in a wide variety of ways.
• Low HRV means your heart rate is very consistent, usually elevated. This indicates your body is working overtime, exerting a consistent focused effort to recover. This can be recovering from stress, exercise, alcohol, travel, sickness, or even sleep deprivation. Low HRV = a consistently high heart rate = still recovering.

Once your body has recovered, it can return to normal, allowing for a wider range of activity and variability in heart rate. When you are fully recovered, you'll experience higher HRV again.

For this reason, HRV is a solid measure of overall recovery.

One last point

HRV values vary from person to person. What’s considered low HRV for you might be high for someone else, and vice versa. So, it's important to compare your HRV to your own previous values (and their average), rather than to your friend’s.

This was a gross oversimplification of the topic, but a useful introductory framework. For a deeper dive - these two papers are great: Paper 1, Paper 2.

Beyond plain English - how is HRV calculated?

To calculate HRV, you first need to measure the timing of heartbeats (a.k.a. heart rate). The three most common methods for extracting heart rate are ECG, PPG, and BCG. Electrocardiography (ECG or EKG) measures the heart’s electrical activity using voltage sensors. Photoplethysmography (PPG) measures the pulsatile flow of blood through the arteries using an optical sensor. Ballistocardiography (BCG) measures the movement of the skin caused by the pulsatile flow of blood using an accelerometer.

Once the heartbeats are detected, the instantaneous heart rate (the time between two beats) is determined. These instantaneous heart rates are collected over a 5-minute window, and the root mean square of the standard deviation (RMSSD) of these heart rates is calculated. While there are many ways to measure HRV, the 5-minute RMSSD has become the industry standard for wearables in recent years, as it is used by most wrist-worn and finger-worn devices. This 5-minute RMSSD is often averaged across the entire night to provide your daily HRV value.

Note that all you need to calculate HRV using the BCG method is a sensitive-enough accelerometer. The good news is that Elemind’s sleep on-demand headband just so happens to have an ultra sensitive accelerometer in it.  So, as we release new features, keep an eye out for HRV as a potential part of Elemind’s future sleep offering.  

This article was originally published to The Elemind Blog.

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