An Introduction to Heart Rate Variability
Heart rate variability (HRV) is a measure of variation in time (milliseconds) between your heartbeats. HRV provides a snapshot into how your body is balancing between the two branches of your autonomic nervous system: your sympathetic (“fight-or-flight”) and parasympathetic (“rest-and-digest”).
Normal HRV can range anywhere from below 20 to over 200 milliseconds, depending on various factors such as age, gender, physical fitness, and genetics. The HRV value provided by Oura falls within this same range. HRV differs for everyone, so be sure to compare changes in your HRV to your baseline and not others. Oura supplies you with a personalized baseline so you know what your normal HRV is.
As a general rule:
• High HRV (relative to your baseline) is associated with activation of your “rest-and-digest” branch, general physical fitness, and good recovery. Higher average nighttime HRV measurements have also been linked to better sleep quality and vice-versa.
• Low HRV (relative to your baseline) is associated with activation of your “fight-or-flight” branch, stress (both good and bad), illness, and overtraining.
The Importance of HRV
You have an autonomic nervous system that acts unconsciously and is responsible for regulating key bodily functions such as heart rate, digestion, blood pressure, and breathing, among other things. It is divided into the two branches introduced above: sympathetic (“fight-or-flight”) and parasympathetic (“rest-and-digest”). Heart rate variability is produced by competing inputs from these two branches.
The sympathetic nervous system is what tells your body to “go” and the parasympathetic nervous system is what tells your body to slow down and recover. When both of these branches are giving inputs to your heart, you’ll see increases and decreases in your heart rate, which naturally causes variation between beats. Heart rate and heart rate variability tend to be inversely connected. Having a slower heart rate means that there is more room for variability between each beat, which typically results in a higher HRV. In contrast, having a faster heart rate limits space for variability between each beat, which usually results in a lower HRV.
Having variation between your heartbeats is a positive thing because it demonstrates that your autonomic nervous system is balanced and capable of responding to a wide variety of stimuli in a healthy manner. This is essential because there will always be activating and deactivating signals coming from the world around us, as well as inside our own bodies. Our bodies react to both positive and negative stimuli relentlessly. Whether it’s fighting off a virus, overcoming a poor night of sleep, encountering sounding horns in the middle of traffic, or seeing a friend for the first time in a while, our nervous systems are constantly being put to the test. The stronger your autonomic nervous system is, the easier you can adapt in situations where unexpected stimuli are thrown your way.
How Oura Measures HRV
Oura calculates your nighttime HRV using rMSSD, a well-known HRV parameter that provides a reliable and accurate view of your autonomic nervous system’s activity. The average HRV you see on your Readiness tab is the mean of all 5-minute samples measured while you sleep. What’s unique about Oura is that changes in your HRV are accounted for every 5 minutes throughout the night in comparison to other wearables that only take HRV measurement at a single point during the night. Having continual measurements throughout the night not only results in greater accuracy but gives you a more detailed perspective into how your body is responding to the previous day’s stressors and how well it’s preparing for the next.
As displayed in the Readiness tab view shown below, Oura provides you with the following heart rate variability metrics:
• Average HRV: Your average HRV captured over the entire night
• Max HRV: Your highest HRV value captured over the entire night
• Your HRV Trace: Your HRV values captured every 5 minutes throughout the night, displayed in a graph.
You can find this data in your Readiness tab, as well as in Trends. Trends can be found by tapping on the menu icon (☰) located in the upper left-hand corner of your home screen. Please see our article on using Trends for more information on how to get started with this feature.
Understand Your HRV in the Long-Term
HRV can’t get rid of your stress or improve your cardiovascular fitness on its own, but it can offer insight into how your behavior, environment, emotions, thoughts, and feelings are impacting your bodily functions. In this sense, measuring and tracking HRV can be the first step necessary to jumpstart a healthier lifestyle: whether that be helping you to respond to stress in a more meaningful way, building more physical activity into your weekly routines, or bringing awareness to how your diet selections may be placing your body into fight-or-flight mode, monitoring your HRV is an amazing place to start.
If you’re interested in evaluating your HRV for long-term changes, we suggest you read more about HRV balance, which happens to be one of your Readiness contributors.
Check out The Pulse article, “What is HRV Balance” for everything you’ll need to know about this powerful metric.
Interpret Your HRV in the Short-Term
1. Interpret Your HRV Trace
• Higher HRV values tend to occur during REM sleep.
• Lower HRV values tend to occur during deep sleep.
2. Interpret Your HRV Values (High & Low)
HRV levels that are high or slightly higher than your normal range tend to be signs of good recovery. A higher HRV may be the result of:
• A much-needed rest day
• A cool bedroom at night
• Participation in more mindful, low-to-moderate-intensity activities like hiking or yoga
• Engagement in mindfulness meditation
HRV levels that are lower than your normal range are signs of excessive strain on the body. Lower HRV may be the result of:
• Consumption of alcohol
• A late night meal or workout
• Acute and prolonged stress
• A hot bedroom at night
• Jet lag and inconsistent sleep patterns
*Note: naturally, after a hard workout, your HRV will likely decline. As you recover, your HRV should rebound. If it doesn't return to your baseline or takes an extended period of time to, this can be an indication that you’re training too often or too hard.
Considerably higher HRV levels are not necessarily a good thing. These may be an indication of low autonomic nervous system stimulation, which can be felt as low energy and lethargy. Overly high HRV scores are a sign from your body that you’re ready to take on a physical challenge to build cardiovascular fitness in pursuit of greater health.
When Does Oura Measure Your Heart Rate Variability?
Oura doesn’t track heart rate or heart rate variability during exercise or other activities during the day. By focusing on measuring heart rate and heart rate variability during sleep only, Oura is able to block out confounding “noise” during daytime hours that would otherwise impact your data and make them challenging to interpret. Heart rate and heart rate variability are two very sensitive metrics, meaning they’re easily subject to change via activities as simple as drinking a glass of water, getting up to go to the bathroom, or watching an exciting television show.
The idea behind Oura is to present data that is meaningful and actionable, which is why we focus on measurements during sleep. Sleep is the most stable state your body will be in during a given 24-hour period. Without external stimuli from food, water, personal interactions, the environment, or movement, Oura is able to capture a crisp and accurate snapshot into how your heart and nervous system are operating on their own.
The only exception to this is while using Moment, which allows you to meditate or check-in with your body. Certain Moment sessions, upon completion, show you a snapshot of your resting heart rate during the day. This allows you to observe the potential benefits of engaging in mindfulness meditation.
To learn more about HRV, check out The Pulse.