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Understanding Heart Rate Variability (HRV)

“Anyone wanting to change their lifestyle habits: lose weight, control stress, monitor its wellness, training or going for a marathon should track their daily HRV. It is an accurate and powerful tool that none should ignore.” Amaury Kosman, Circular CEO.

Heart rate variability (HRV) gets a lot of attention nowadays since it is a powerful and reliable metric. It has uses for athletes to optimize their training, for people wanting to reduce their stress and for people to check on their overall wellness and recovery. It is studied globally for its correlation to our autonomic nervous system. 

The basics: what is HRV?

HRV measures the specific changes in time (or variability) between successive heartbeats. The time between beats is measured in milliseconds (ms) and is called an “R-R interval”.

R-R interval Circular

A healthy heartbeat contains healthy irregularities. If your beats per minute is 60 for example, your heart will most likely not beat once every second for a whole minute. It will rather have different interval lengths between each heartbeat. One interval could be 0.75 seconds for example and the next one could very well be 1.25 seconds.

To summarize HRV as an easy way to understand, HRV should be seen as an index that allows you to know if your heart is able to adapt to your activities. If you have a high HRV then it means that your heart shows high variation and therefore it can beat slowly or faster according to its needs. If you have a low HRV, it shows that there is very little variation and that your heart is beating in a constant manner. In this case, if you have to run, then your heart will have a hard time keeping up and you will get tired much faster. In conclusion your HRV demonstrate your ability to react to your environment. Generally speaking, if the intervals between your heartbeats are rather constant, your HRV is low. And if their length variates, your HRV is high.

As a micro view, HRV even varies according to our breaths. It decreases when we inhale and increases when we exhale.

How is HRV measured?

The heart rate, even without a tracking device , can be felt. However HRV is impossible to assess without a measuring device.

Each contraction of the heart results in a blood volume pulse which propagates through the bigger arteries towards the small capillaries. We can all feel this by placing a finger on top of our arteries on our neck or the palmar side of our wrist.

HRV measurements are made easy to track with Circular. This blood volume pulse signal can be tracked optically. Optical measurement is based on the absorption of certain wavelengths of light when reflected towards blood veins. In this case, we talk about (PPG) measurement.

PPG Circular Ring

This method allows for non-invasive HRV tracking methods such as… a ring! 

A great benefit of having a device like the Circular ring on your finger is that it is most likely one of the most optimal places for PPG-based HRV tracking. The reason being that it has suitable arteries and capillaries for clear blood volume pulse signals, making the optical measurement more reliable and accurate. In addition, there are no moving parts between the joints in fingers, meaning that the ring sits firmly and doesn’t move. 

HRV is measured within a specific timeframe. At Circular we use the RMSSD calculation. The measurement time frame is a 1 minute average.

So how can HRV help you?

Circular automatically detects your reference HRV which is your long term average when you feel ordinarily. Then Circular makes the average of your sleep HRV readings to give one average of your daily HRV. When your daily HRV is greater than or equal to your reference HRV then you are good to push yourself. When your daily HRV is lower than your reference then you need to keep it cool.

It is important to understand that HRV only must be compared to your own trends and reference because it is unique to you. There are a couple of main applications for its use:


In recent scientific literature, an experience was made where a group of runners performed better when doing intense training only under good physiological conditions, despite having actually trained less than the other group that could train under any conditions. Good physiological conditions meant having their daily HRV within their normal values or trending positively compared to their reference. A negative trend was a no go. This shows the importance of training intelligently rather than quantitatively.

We aim at using HRV to quantify recovery. There is quite a strong relationship between intense aerobic workouts and reductions in HRV on the following day. After a workout, your HRV tends to go down if your training was intense but should return to normal and even higher once you have recovered. This is a typical acute stressor, and the reduction in HRV (and a smaller increase in HR) can be used to quantify recovery and understand if you need an extra recovery.

High HRV, which shows larger gaps between heartbeats shows positive adaptation and fitness while lower HRV with smaller gaps indicates fatigue and overtraining. 

Knowing your heart rate variability means understanding your body and its response to physical overload so you can tailor your training regime for optimal results.

Wellness Analysis 

It has been proved that the healthier you are, the higher the variation between heartbeats is. In other words, the healthier you are, the higher your HRV is. People who have high HRV may have greater cardiovascular fitness and be more resilient to stress. 

Over the past few decades, research has shown a relationship between low HRV and worsening depression or anxiety. A low HRV is even associated with an increased risk of cardiovascular disease.

HRV also provides personal feedback about your lifestyle and is the best tool to quantify wellbeing. If you consider yourself as taking steps toward a healthier life, you should consider tracking your HRV. It's much more valuable than your bpm.

It is fascinating to see how HRV evolves as you do your activities, meditation, sleep, and especially physical. For those who love data, this can be a nice way to track how your nervous system is reacting not only to the environment, but also to your emotions, thoughts, and feelings.

If you are sick, you have done intense physical training, or if you have slept badly you will definitely see the impact on your HRV.

Stress Analysis

Heart rate variability is an excellent, non-invasive way to measure stress. Stress is an important metric to track as many people don’t recognize the signs and symptoms, which often leads to deeper health issues if left unmanaged. Studies have shown that changes in heart rate variability are tied to a variety of health problems, from heart diseases, diabetes to post-traumatic stress disorder (PTSD).

Improving Meditation

Higher HRV is a strong biomarker for general health and resilience as high HRV indicates a relaxed, low-stress mind, while lower HRV suggests the need for sleep and rest. As meditation requires a calm and relaxed mind, measuring heart rate variability is an effective way to improve your meditation state. Especially since HRV and meditation are closely linked because there are meditation and breathing techniques to improve HRV in the short and long term.

Anxiety Treatment

It has been shown that anxiety disorders are associated with low heart rate variability values. Anxiety disorders are the most common psychiatric disorders today and have been shown to increase the risk of heart disease. So monitoring heart rate variability of anxiety patients is important as changes in HRV reflect the effectiveness of treatment.


As a whole, high HRV is an indication of overall health as well as general fitness. Generally speaking, it tells us how recovered and ready we are for the day. HRV reacts to changes in our body earlier than heart rate and this makes a particularly sensitive tool that gives us insights into our wellbeing. HRV will also help you detect you are getting weak before you even get the symptoms of a sickness. 

Remember that the Circular ring and Kira do the work for you and will alert you based on your unique profile for HRV changes. But it is also nice to sometimes manually check your HRV readings and understand how your body reacts to your environment.

You can now learn about how to use Heart Rate Variability (HRV) with Circular 


Aside from that, there are multiple studies indicating that HRV is quite useful as a way to quantitatively measure physiological changes caused by various interventions both pharmacological and non-pharmacological during treatment of many pathological conditions having a significant manifestation of lowered HRV.

However, it is important to realize that the clinical implication of HRV analysis has been clearly recognized in only two medical conditions:

1. Predictor of a risk of arrhythmic events or sudden cardiac death after an acute heart attack

2. Clinical marker of diabetic neuropathy evolution

Nevertheless, as the number of clinical studies involving HRV in various clinical aspects and conditions grows, HRV remains one of the most promising methods of investigating general health in the future. 

Scientific sources:

  • Opinion: “Heart Rate Variability, Health and Well-Being: A Systems Perspective” Research Topic. Angela J. Grippo
  • Heart rate variability: a review in Medical & Biological Engineering & Computing, Rajendra Acharya, K. Paul Joseph, N. Kannathal, Choo Min Lim, Jasjit S. Su, January 2007
  • Heart rate variability and myocardial infarction: systematic literature review and metanalysis. European Review for Medical and Pharmacological Sciences, Buccelletti E1, Gilardi E, Scaini E, Galiuto L, Persiani R, Biondi A, Basile F, Silveri NG. July-August 2009.
  • Reduced heart rate variability and mortality risk in an elderly cohort. The Framingham Heart Study. Circulation, Tsuji H1, Venditti FJ Jr, Manders ES, Evans JC, Larson MG, Feldman CL, Levy D. August 1994.
  • Influence diagram of physiological and environmental factors affecting heart rate variability: an extended literature overview, Heart Int. Julien Fatisson, Victor Oswald and François Lalonde. 2016 Jan.

We remind you that the Circular™ ring is not a medical device and should not be used to diagnose or monitor a pathology.

Laurent Bsalis

Laurent is passionate about biohacking and tries his best to be a better version of himself. He also eats too much ice cream which is a problem.

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