HRV stands for Heart Rate Variability. Researchers and physiologists have been tracking and utilizing HRV for decades because it’s a useful indicator of several health-related issues (more about these later), but only lately has it grabbed the attention of athletes, coaches, biohackers and the general public.
The Oura ring tracks your HRV during the night which makes it an especially useful tool for tracking your HRV with little extra effort.
On the left, the Oura app draws your night-time HRV curve, as well as average and max HRV. On the right, your long-term HRV trend.
We’ll cover the basics of heart rate variability in two blog articles. This article introduces you to HRV in general. You’ll learn:
This second article in the series digs deeper into the science of HRV measurement and explains how and why Oura tracks HRV.
A healthy heart beat contains healthy irregularities. Even if your heart rate is, say, 60 beats per minute, that doesn’t mean that your heart beats once every second – or at one-second intervals like a clock.
Rather, there is variation among the intervals between your heartbeats. The interval between your successive heartbeats can be, for example, 0.85 seconds between some two succeeding beats and 1.35 seconds between some other two.
Even though the difference is measured in parts of seconds, you can actually feel the difference.
Here’s a tip for anyone who wants to experience it: place a finger gently on your neck or wrist and find your pulse. You should feel that the longest intervals take place when you exhale, and the shortest intervals when you inhale.
How you calculate heart rate variability depends on what technology you use. Using an ECG, or electrocardiogram, it’s typically the R peak in the QRS complex that marks a heartbeat. Hence, the intervals between heartbeats are called R-R intervals.
With Oura, your heartbeats are analyzed with PPG (photoplethysmography). In the PPG signal it’s the steepest increase in the signal prior to the peak that marks a heartbeat. Instead of R-R intervals, we measure interbeat intervals, or IBIs. See the image below for what ECG and PPG signals look like.
Heart Rate Variability is a measure which indicates the variation in your heartbeats within a specific timeframe. The unit of measurement is milliseconds (ms). For Oura, the measurement timeframe is 5 minutes.
There are different ways to calculate HRV, but they all have to do with the amount of variation in the intervals between heartbeats. Oura utilizes rMSSD (Root Mean Square of the Successive Differences), which is the most commonly used HRV formula. The figure below is an example of interbeat intervals in milliseconds.
Why track HRV? The key is the phrase “healthy irregularities” we used in the beginning of the article. The type of variability indicated by HRV is perfectly normal. Actually, it’s desirable.
To understand HRV, we first need to understand our nervous system and heart rate. Heart rate variability can be traced back to our autonomic nervous system.
The autonomic nervous system regulates very important systems in our body, including heart and respiration rate and digestion. The autonomic nervous system has a parasympathetic (rest) and a sympathetic (activation) branch. Heart rate variability is an indicator that both branches are functioning – the parasympathetic in particular.
Intrinsic heart rate is measured in the condition in which neither parasympathetic nor sympathetic regulation is present. When completely blocked from autonomic regulation, a healthy heart contracts at a rate of about 100 beats per minute (the number is individual, however).
Parasympathetic regulation lowers your heart rate from the intrinsic level, giving more room for variability between successive heartbeats. Parasympathetic regulation causes almost immediate changes that affect only a few beats at a time, after which the heart rate returns towards the intrinsic rate. Sympathetic regulation elevates your heart rate from the intrinsic level, and there is less room for variability between successive heartbeats. Sympathetic regulation affects several consecutive heart beats.
Put these together and we can formulate a rule that when the rest-related parasympathetic branch is active and the sympathetic branch is inactive, your heart rate is lower and HRV higher.
Put these together and we can formulate a rule that when the rest-related parasympathetic branch is active and the sympathetic branch is inactive, your heart rate is lower and HRV higher. Factors such as stress can lead to the withdrawal of parasympathetic activity, or activation of sympathetic branch even when you are resting, both leading to elevated heart rate and lowered HRV.
Heart rate variability has been studied, for example, in the context of cardiovascular training.
When you start regular cardiovascular training, one of the fastest positive adaptations of your body is increased blood plasma volume, and subsequently increased stroke volume. As a result, your heart can keep the blood flowing and maintain adequate blood pressure at a lower heart rate. And as we remember, lower heart rate is regulated by the parasympathetic branch. Parasympathetic regulation causes longer interbeat intervals and elevated HRV.
In the long term, regular exercise also strengthens the heart muscle, which once again means lower HR and higher HRV.
On the whole, high heart rate variability is an indication of especially cardiovascular, but also overall health as well as general fitness. Generally speaking, it tells us how recovered and ready we are for the day. Also, HRV can react to changes in our body even earlier than heart rate. This makes it a particularly sensitive tool that gives us insights into our wellbeing.
You shouldn’t compare your heart rate variability with other people, because HRV is affected by a number of internal and external factors such as age, hormones and the overall body functions, as well as lifestyle. Furthermore, at a given heart rate, women typically have a higher heart rate variability than men.
There are no generic guidelines for optimal HRV values – which is understandable considering there are several ways to both track and calculate it. The HRV value given by Oura (rMSSD5min) can range from anywhere below 20 to over 100 ms. HRV tends to be higher when you’re fit and healthy, but how high is high depends on the individual.
You shouldn’t compare your heart rate variability with that of other people.
Instead of comparing your HRV values with someone else’s (even people of same age and gender), you should concentrate on your own HRV and its trends. Also, for your daily HRV values to be comparable with each other, they should be tracked with the same method and in similar conditions.
Interested in tracking your heart rate variability? The award-winning Oura ring with the Oura app is one of the easiest, most comfortable ways to track HRV in the long-term.
Oura calculates your HRV for each night. Comfortable nighttime tracking means that the conditions for measurement remain the same. Consequently, you can easily compare your values, both in the short and the long term, and you don’t need to do anything else except wear the ring.
Heart rate variability is one of the indicators of the state of your health and fitness, recovery and readiness. However, your HRV values, like your overall health and fitness, are a combination of several things, so pay attention to yourself and how you feel as a whole. HRV is a good indicator, but it’s still just one indicator. Don’t rely too much on it, or any other measure alone.
With that out of the way, here are some ways you can learn from your HRV values.
The first thing to pay attention to is your own HRV baseline. That is, your typical HRV when you’re feeling as you feel on average. Your baseline is the starting point for your HRV explorations. You will get an understanding of your HRV baseline after using the Oura ring for a while, because Oura shows you both the nightly HRV value and the long-term HRV trend.
After discovering your baseline, you’re ready to follow up how your lifestyle and health affect your HRV. If your HRV goes down, something might be burdening your body and/or mind. If your HRV goes up, something might be doing good for your body and/or mind.
Before increasing your training load significantly or making major changes to your nutrition, remember to consult your physician – especially if you don’t feel well or if you suffer from long-term illnesses.
The Oura app and Oura Cloud show you the previous nights’ HRV, indicating your current HRV status. You can also see your weekly and monthly HRV averages, which help you to create a bigger picture of your readiness in the long term.
Below are some practical examples of how changes in HRV can be interpreted, both in the short and the long term. Bear in mind that you and your HRV are unique, so all the examples may not be applicable to your situation.
Average heart rate variability and lowest resting heart rate before (Mon 28 – Wed 30), during (Thu 31 – Tue 05) and after (Wed 06 – Sat 09) illness
If you’re interested in tracking your HRV but don’t have the tools for it, have a look at the new Oura ring in the Oura Shop. If you have an Oura ring, you can dig deep into your HRV data in the Oura Cloud.
In this second part of our guest blog series, our prestigious visiting writer Dr. Benjamin Smarr tells you about chronotypes – our genetically shaped circadian preferences – and their effect on our everyday life.