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Everything You Need to Know About Heart Rate Variability (HRV)

Heart rate variability, or HRV for short, is a measure of your autonomic nervous system that is widely considered one of the best objective metrics for physical fitness and determining your body’s readiness to perform.


Heart rate variability is literally the variance in time between the beats of your heart. So, if your heart rate is 60 beats per minute, it’s not actually beating once every second. Within that minute there may be 0.9 seconds between two beats, for example, and 1.15 seconds between two others. The greater this variability is, the more “ready” your body is to execute at a high level.

These periods of time between successive heart beats are known as RR intervals (named for the heartbeat’s R-phase, the spikes you see on an EKG), measured in milliseconds.

For instance, the interval between heartbeats is generally longer on your exhales and shorter when you inhale. So even if your heart rate is 60 beats per minute, the time between these beats is rarely one exact second. Within the same minute, you could have a 0.8-second interval between one set of heartbeats and then a 1.13-second interval between another set.

This probably seems overly scientific and perhaps not that useful for the everyday amateur athlete. However, being able to answer the question ‘what is HRV?’ can in fact provide you with important information about your overall health and the progression of your current training plan.

RR intervals show heart rate variability.

WHOOP calculates HRV using RMSSD, the root mean square of successive differences between heartbeats.


When you have high heart rate variability, it means that your body is responsive to both sets of inputs (parasympathetic and sympathetic). This is a sign that your nervous system is balanced, and that your body is very capable of adapting to its environment and performing at its best.

On the other hand, if you have low heart rate variability, one branch is dominating (usually the sympathetic) and sending stronger signals to your heart than the other. There are times when this is a good thing–like if you’re running a race you want your body to focus on allocating resources to your legs (sympathetic) as opposed to digesting food (parasympathetic).

However, if you’re not doing something active, low HRV indicates your body is working hard for some other reason (maybe you’re fatigued, dehydrated, stressed, or sick and need to recover) which leaves fewer resources available to dedicate towards exercising, competing, giving a presentation at work, etc.

To look at it another way, the less one branch is dominating the other, the more room there is for the sympathetic (activating) branch to be able to come in and dominate, which is why high HRV suggests you’re fit and ready to go.

There are a variety of factors that can influence or change your HRV metrics. These include:

  • Respiration
  • Exercise volume and intensity
  • Rest and recovery from workouts
  • Hormones
  • Metabolism
  • Chronic health conditions
  • Stress
  • Sleep habits and quality of sleep
  • Diet
  • Age and gender
  • Genetics

Another reason for this great variance in heart rate is because of our nervous system. Your parasympathetic nervous system receives input from your organs, which can cause a decrease in your heart rate. On the other hand, your sympathetic nervous system reacts to outside factors like exercise and stress and can increase your heart rate.

These simultaneous signals that two branches of our nervous system send to our heart cause it to constantly fluctuate, directly influencing your heart rate variability numbers.


Below is an average heart rate variability chart based on age:

A heart rate variability chart ms, showing the middle 50% average by age of all WHOOP members.

You can see that for the most part, HRV decreases abruptly as people get older. The middle 50% of 20-25 year olds usually have an average HRV in the 55-105 range, while 60-65 year olds tend to be between 25-45.


As we have seen, heart rate variability can depend on a number of different factors. This makes it a highly individualized metric that can be difficult to compare from person to person. Instead, monitoring this data for trends in your own heart rate variability baseline metrics can be more useful. 

That being said, high heart rate variability numbers can be an indicator that your heart is functioning well, adapting to environmental stressors, and seeing an increased level of fitness. Keep in mind though that this metric can be sensitive, and fluctuate from one day to the next.

In general, younger people tend to have higher HRV numbers than older people, males will often have higher numbers than females, and elite athletes will have higher numbers than just about everyone else.


Your general health and wellbeing have a strong influence on your lifespan, happiness, and quality of life. Keeping track of your heart rate variability metrics can provide surprising insights, such as tracking your fitness levels or understanding how outside factors could be having a negative impact. 

For example, poor nutrition, insufficient sleep, illness, or an increase in stress could all cause your HRV numbers to lower. By monitoring this metric, you can make adjustments as needed to address these issues so that your health does not begin to decline.

For runners, cyclists, or fitness enthusiasts, heart rate variability can help you track the progression of your training. This includes how rested you are from a workout and how well your body will be able to tolerate another hard session. 

A good example of this is how low heart rate variability often occurs after a hard or strenuous workout. With proper rest and recovery, you should see your numbers rise again in a day or two – an indication that you are ready for another hard workout.

As you gain cardiovascular fitness, you should start to see your heart rate variability slowly improve. If you still have a low HRV metric, it could be an indication that you’re not getting enough rest and recovery, or need to adjust your training plan. Using this metric to your advantage can help you train more efficiently and get the most out of your fitness regimen without risking injury or illness.



Improving your cardiovascular health, endurance, and heart rate variability all go hand in hand. For those looking to improve this metric for health or performance reasons, here are some basic tips you can use to improve your heart rate variability metrics.


Training hard on too many consecutive days without a day of rest or recovery can leave your body feeling run down. For this reason, schedule active recovery workouts or recovery days following a hard block of training.


Being hydrated can improve circulation and make it easier for your blood to deliver oxygen to the body. This can have a direct impact on your training, health, and overall stress.


Did you know that alcohol can affect your heart rate variability for as long as five days? Poor nutrition can also have a similar effect on your data and impact how your body feels while working out.


Your body will adapt to your training load more easily when you’re consistent. This can involve things like working out at the same time of day, having a set sleep schedule, and avoid those random binge nights with junk food on the weekends.

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