What Is Heart Rate Variability And What You Can Learn From It

Heart rate variability has lately gained the attention of many people looking to track, understand and optimize their recovery and readiness. Heart rate variability or HRV is something you can track with the Oura ring but what is HRV all about and why should we care?

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.

What You’ll Learn In This Article

We’ll cover the basics of heart rate variability in two blog articles. This article introduces you to HRV in general. You’ll learn:

  • What HRV is
  • What HRV tells you about your body
  • Why you shouldn’t compare your HRV to anyone else’s HRV
  • What you can learn from tracking your HRV

An upcoming second article will dig deeper into the science of HRV measurement and explains how and why Oura tracks HRV.

What Is Heart Rate Variability (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 e.g. 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.

R-R Intervals and Interbeat Intervals

How you calculate heart rate variability depends on what technology you use. Using 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.

ppg and ecg signal

Defining Heart Rate Variability

Heart Rate Variability is a measure which indicates how much variation there is in your heartbeats within a specific timeframe. The unit of measurement is milliseconds (ms). For Oura, the measurement timeframe is 5 minutes.

  • If the intervals between your heartbeats are rather constant, your HRV is low.
  • If their length variates, your HRV is high.

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.

ppg graph

Why Is Heart Rate Variability an Important Measure?

Why track HRV? The key is the phrase “healthy irregularities” we used in the beginning of the article. The type of variability HRV indicates is perfectly normal. Actually, it’s desirable.

The Basics

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 or 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, though).

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 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.

HRV and Cardiovascular Training

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.

Your Heart Rate Variability Is Unique

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 that 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.

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.

What Can You Learn From Your HRV?

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.

Remember to consult your physician especially if you don’t feel well, before increasing your training load significantly, or making major changes to your nutrition, particularly 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.

Practical Examples

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.

Heart rate curve during illness

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

  1. You might notice that a very intensive endurance exercise can acutely lower your HRV, but if you recover well, it will jump back up again soon. This is generally speaking a sign that your body handles the training load well. If your HRV doesn’t jump back up, you may have been training too hard or too often. Overall, it seems that regular endurance exercise does tend to increase HRV in the long run, and a high HRV value (compared to your baseline) means that your body should be ready for the next exercise.
  2. If you become stressed or don’t sleep well for a while, you could see your HRV values gradually dropping, indicating that your body might not be at its best, and you need to take some time for recovery.
  3. One interesting finding is that your HRV can jump down if you’re about to get sick – even before you develop any symptoms. If this is the case, and you can take it easy for a day or two, your body might fight off the disease. If you get sick, your HRV can stay quite low even after the symptoms are gone. This indicates that your body is still recovering, and isn’t ready for maximal performances.
  4. Smoking and alcohol consumption can reduce HRV. So, you will most probably notice that your HRV goes down momentarily after a night out in town.
  5. If you’re an endurance athlete or otherwise exercise-oriented, these research findings* might interest you: among recreational endurance athletes, it seems that the level of HRV predicts training response for several weeks of training. If you have high HRV, your body can be expected to respond positively to high intensity training. If you have low HRV, your body can be expected to respond positively to high volume of low intensity training.
  6. Overall, HRV drops with dehydration, but jumps back to its baseline level with good hydration. As discussed, both exercise and alcohol can cause dehydration – as can long bouts in the sauna (for all you Finns out there!).

If you’re interested in tracking your HRV and you don’t yet 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.

*
Hautala AJ, Kiviniemi AM, Makikallio TH, Kinnunen H, Nissilä S, Huikuri HV, Tulppo MP. Individual differences in the responses to endurance and resistance training. _Eur J Appl Physiol_ 2006;96(5):535–42.

Vesterinen V, Häkkinen K, Hynynen E, Mikkola J, Hokka L, Nummela A. Heart rate variability in prediction of individual adaptation to endurance training in recreational endurance runners. Scand J Med Sci Sports 2013;23(2):171–80.

Vesterinen V, Häkkinen K, Laine T, Hynynen E, Mikkola J, Nummela A. Predictors of individual adaptation to high-volume or high-intensity endurance training in recreational endurance runners. Scand J Med Sci Sports 2016;26(8):885–93.

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