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Understanding Heart Rate Variability (HRV)

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

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.

The healthiest you are, the more your heart must be able to adapt to any situation, to accelerate as well as to slow down, that’s why your R-R interval will be irregular. If it is more regular it means that you are out of shape and that your heart will not be able to easily adapt.

RMSSD calculation method which is the root mean square of successive RR interval differences is often used and will give you an easier score number to study. The higher the score the better you are, the lower the score the less healthy you are.

HRV is measured within a specific time frame. For Circular, the measurement time frame is 5 minutes. Generally speaking, if the intervals between your heartbeats are rather constant, your HRV is low. And if their length variates, your HRV is high.

How is HRV measured?

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. 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 the 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. 

So how can HRV help me?

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

Heart Rate Variability Circular

It is important to understand that HRV is an interesting metric to compare to your own trends and baseline because it is unique to you. There are a couple of main applications for its use that can help you out:


In recent literature, a group of runners performed better when starting intense training blocks only under good physiological conditions, despite having actually trained less than the control group. Good physiological conditions meant simply having their baseline HRV within their normal values or trending positively. A negative trend was a no go.

We aim at using HRV to quantify recovery. There is quite a strong relationship between intense aerobic workouts and reductions in HRV the following day. 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 we need an extra day off. 

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, in other words, 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 death and cardiovascular disease.

HRV may also provide personal feedback about your lifestyle and help motivate those who are considering taking steps toward a healthier life.

It is fascinating to see how HRV evolves as you do your activities (especially physical), as you meditate and as you sleep. 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 trained physically, 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 Heart Rate Variability 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, relaxed mind, measuring heart rate variability and training for a higher HRV is an effective way to improve your meditation state.

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 a treatment.


On the whole, high Heart Rate Variability 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. 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. This can help you detect if you are getting weak before you get sick. 

Remember that Circular 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 HRV readings and understand how your body reacts to it yourself.

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 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 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:

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

The Circular ring: continuous Heart Rate monitoring

The ring has an optical infrared & red pulse sensor that allows you to record and display in real-time and for later analyses/recommendations on the App. 

How is Heart Rate measured?

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. The same happens on your finger.

The optical heart rate sensors use a methodology called photoplethysmography (PPG). PPG is a technical term for shining light into the skin and measuring the amount of light that is scattered by blood flow. That is a bit of an oversimplification, but PPG is based on the fact that light entering the body will scatter in a predictable manner as the blood flow dynamics change, such as with changes in blood pulse rates (heart rate) or with changes in blood volume (cardiac output).

In PPG recording, you don’t need electrodes, but just a device that has a proper LED and a photo receiver. That’s why it’s a common method used in wearables, such as in the Circular ring.

Infrared light travels deeper into our tissues and enables more accurate measurements than other wavelengths of light such as green or red light.

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

Live Heart Rate Circular

How can Heart Rate help me?

Heart Rate is the main component alongside to your movements for every Circular sleep, activity and wellness analyses.

Here is a list of the different useful factors that derive from your Heart Rate.

Heart Rate Variability

HRV then leads to sleep analyses, respiration rate, wellness analyses, stress and recovery analyses.

To know more about HRV: Understanding Heart Rate Variability (HRV)

Rested Heart Rate

RHR is an excellent indicator of your overall wellness performance, to be correlated with your HRV.

To know more about RHR: What increase and decrease Rested Heart Rate (RHR)?

Maximum Heart Rate

MHR then leads to heart training zone.

To know more about MHR: Why Max Heart Rate (MHR) is important to know

Activity intensities

Activity intensities describe the types of effort that you produce (which trigger different results) to focus on efforts that match your performance goals.

To know more about Activity intensity: How Circular tracks your Activity intensity

VO2 Max

VO2 Max is an excellent indicator of your potential performance in endurance events.

To know more about VO2 Max: Understanding VO2 max: why is it useful?

Calories burned

Calories burned gives you the power to make informed decisions about nutrition, weight management, and performance.

To know more about Calories burned: Circular’s calories burned


SpO2 helps you keep track of your blood oxygen saturation levels. May be useful for high altitudes activities, people suffering from asthma or other oxygen conditions.

To know more about SpO2:Oxygen Saturation (SpO2) and how to use it to your advantage 

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

Best practices from a high-performance coach and bio-hacker: Siim Land


Waking up sounds the beginning of the day and you know the importance of a good wake up. You even wrote an e-book called “Waking up Empowered”. Why do you believe it to be so important?

“The way you wake up determines how the rest of the day is going to go. If you start ruminating over negative thoughts or hit the snooze, then you’re telling your brain what kind of a rhythm to follow. On the other hand, if you get up doing something productive such as meditation or making your bed, you’re dragging momentum to your side and everything else gets easier as well.

There’s a reason people use the idiom of stepping out of bed with the wrong foot. Your brain is plastic and always listening to the feedback it receives from your thoughts, emotions, and actions. That’s why I believe in getting quick wins as soon as possible. It can be taking a cold shower, reading a few pages, writing a journal or exercise. Whatever puts you into the flow of things. In any case, train your brain to avoid negativity and procrastination.”

What would be your top tips for a better wake up? How do you personally combat sleep inertia?

“The best tip for a restful wake-up is to wake up naturally without an alarm clock. This way your brain will gradually pull you out of sleep at the lightest stage of sleep and at the end of a sleep cycle. Sleep inertia happens when you get shooked from a deep sleep in the middle of it.

Ideally, you want to not use an alarm clock and stick to a consistent bedtime so your body could develop the habit of waking you up around the same time. If this is not possible, then using specific smart alarm clock or circadian alarm clocks that start calibrating light towards more brightness to mimic the sunrise is also useful.”

What does your routine look like right after waking up?

“After getting clothed I make my bed and go outside to get some fresh air and daylight. This will kickstart the proper circadian rhythm. I may also do some breathing exercises and stretches to get the blood flowing. Then I go inside and start working on my laptop until noon.”


Do you have any sleep hygiene tips that are easily implemented?

“Use blue-blocking glasses to filter out artificial light in the evening. This protects your circadian rhythms and enables the body to produce melatonin the sleep hormone.”

Can you explain the relationship between sleep on activity and better performances?

“Sleep facilitates recovery and adaptation. Without enough sleep, your body wouldn’t be able to repair itself from both cognitive and physical activities. Poor sleep also decreases reaction time, alertness and mood.”

You also talk about sleep and fat loss. How is that directly affected?

“Sleep improves the body’s metabolic flexibility and insulin sensitivity. Sleep deprivation raises blood sugar and cortisol the stress hormone, which makes it easier to get diabetes and gain weight. Psychologically, short sleep also makes you want to eat more food and you’re less satisfied with it.”

Serotonin and thus Melatonin impact our Circadian rhythms and finding the perfect sleep balance is key for a good night’s sleep and to improve our energy. How can one boost his melatonin/serotonin secretion?

“Light exposure affects melatonin the most. That’s why blue blockers and dimming down the lights in the evening are crucial. Certain foods with some carbs and protein will also promote serotonin, which leads to the creation of melatonin. The best foods for that tend to be poultry, meat, bananas, rice, and cherries.”

We’d then be tempted to take melatonin (or other) supplements. Is it a solution in on the long run? Are there natural supplements out there that offer a better alternative?

“Taking supplements is a quick-fix that doesn’t address the underlying cause. It should be used only in some cases like jet lag or poor sleep and not as a staple. Instead, you should focus more on the light environment, food intake and stress management.”

What would be your main takeaways and pieces of advice for better sleep? Are they any gadgets, products or hacks you can recommend?

“In my own experience, filtering out bright lights in the evening is the most effective strategy for getting better sleep. This way you’ll naturally get sleepier. Tracking your sleep with gadgets will also give you more insight into how well you sleep.”


With everything that is said above, we might get conflicting information about which diet to carry out. With your experience and personal findings, what have you found to be the best diet?

“The optimal diet depends on the person’s energy requirements, genetics, lifestyle, and preference. Generally, anything that helps you to lose weight and stay lean will improve your health and sleep better. Overeating even healthy food isn’t a good idea but whole foods that are minimally processed tend to be the healthiest. Most people also undereat protein, which is the most important macronutrient for improving body composition. Hyperpalatable foods that combine fats and carbs together are low in protein, which is why you tend to overeat them. Higher fiber intake will also create more satiety while keeping the overall calories low. Carbs should be calibrated based on exercise intensity and activity levels.

Skipping meals and doing some intermittent fasting has been shown to be effective for improving body composition but it also has many longevity and anti-aging benefits. That’s why I eat only 1-2 times a day and avoid snacking.”

What are the main reasons for trying to stick to such a diet?

“A high satiety diet that helps you to improve your body composition will make your body more metabolically flexible and helps to stay healthy in the modern food environment.”

Have you found any negative impacts of such eating habits? What about positive impacts?

“More energy, easier to stay lean and having more muscle makes my body burn more calories at rest.”

Would you have any advice for anyone trying to start implementing such diets?

“Focus on nutrient density and not calorie density. Chew your food properly in a low-stress environment to avoid overeating.”

To go further with biohacking

If you wish to learn more about how to master your body, develop your mind, build muscle, lose fat, get motivated, achieve your goals, get more energy and increase productivity, check out Siim’s youtube channel here. Don’t hesitate to also follow him on his socials: Facebook, Twitter and Instagram.

You can also check out his website here with an incredibly vast amount of resources with blog posts, podcasts, e-books and much more.

Understanding the Circular® Energy score: Can you push yourself or should you rest?

Circular’s primary goal is to ensure that our users are provided with tools that help them live their lives to the fullest. That’s why we built the Energy score. This is the ultimate score that tells you how much energy you have accumulated for the day. It is made up of many night and day contributors so that you can evaluate your vitality and know every morning whether or not you can surpass yourself or if you may avoid an over-training.

Energy Score Circular

As with all Circular scores, a score equal or above 90 is Excellent, from 80 to 89 is Good and below 80 is Poor.

The energy levels score is a score that is comprised of your last days:

  • Heart rate variability (HRV) 
  • Resting heart rate (RHR)
  • Activity volume
  • Recovery
  • Sleep quality score & balance
  • Wake up score

Energy Score Circular

All these parameters have different coefficients depending on their importance.

You might have already seen recommendations in you feed stating that all of your body signals are excellent and that you should be ready for any challenges. Or on the contrary, Kira might have told you that you should be careful and rest a certain day because your body signals are poor. 

Let’s discuss in depth what is taken into consideration in the Energy score and what are the implications for you. All of the following metrics are considered to determine whether or not you are ready for the day.

Heart rate variability (HRV)

HRV measures the specific changes in time (or variability) between successive heartbeats. It is used to see how well you recover after certain events, it is also a good measure of stress and anxiety and can unveil many things about your overall well being. It makes sense to take HRV into consideration as it is a general sign of a healthy condition. The higher it is the better: your body and immune system are then ready to tackle challenges during an upcoming day whereas the contrary would probably mean you need to rest.

Resting heart rate (RHR)

RHR is also a great measure for your overall health, correlated to your HRV. Although, this one focuses more on your fitness levels and is great at detecting minor heart conditions. A higher (compared to your baseline) RHR indicates poor overall wellness conditions. Coupling it to HRV and recovery gives us insights as to other factors that might affect your readiness such as medication you might be taking, irregular heart rate conditions, dehydration, an increase in temperature or even an increase in emotions.

Activity Volume

Activity volume is used to quantify whether you do enough physical activity according to the World Health Organization literature but also in comparison to your own amount of activity with your own baseline.


Recovery is a very important aspect to take into consideration when checking your overall condition. Circular automatically checks your activity volume in the past 35 hours. It should not exceed 20 minutes of high-intensity activity (or its equivalent 40min of medium intensity activity). As extreme as it sounds, nobody can withstand that much exercising without recovery, even athletes. Even though you are able to, the scientific literature is clear on the subject, it is unhealthy to exceed such amounts without recovery. If you do, it will negatively impact the recommendation you might see on your feed. 

Sleep quality score & balance

It takes a number of contributions into consideration to determine your 2 last nights sleep; whether you slept long enough but also evaluate if it was of good quality. 

Wake up score

Each time you wake up wearing the Circular ring either with or without the smart alarm clock, you can find your wake up score in the “Alarm clock” circle that evaluates in which sleep stage you woke up in. The perfect moment for you to feel refreshed is to wake you up between your REM stage and the beginning of light 1 sleep stage. At this perfect moment, you will receive a 100 score. Then, the score is built to lower depending on how far from that perfect moment you were in when you woke up and also how far you were inside your stage.

What to deduce from the Energy score recommendations

If you get a recommendation saying that all your body signals are in the green: be assured that it means what it means. Every aspect of your body whether it is the immune system, stress levels or activity recuperation are excellent. It means that you can do intense activities, intense mental activities, having minimal risks of getting sick. You can push yourself throughout the day. If that’s what you usually go about, you’ll be able to do just that, but better. Use good days like these to your advantage as they aren’t all that frequent (unfortunately). Don’t push that training or workload back or say “I’ll do it later”, it’s your day! 

If you get a recommendation saying that all your body signals are in the red: you guessed it, don’t push yourself. Kira takes sleep and previous activities into consideration, she knows how much rest or recovery you need. Our advice: If it’s cold be sure to cover yourself, if you feel tired don’t hesitate to take a nap whenever you can and don’t hesitate to go to bed a little bit earlier.

Energy Levels Circular

The Energy score is part of the Circular® Global score, click here to learn more about it.  

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

How Circular tracks your Activity intensity

Circular tracks your activity with the assumption that there is a close linear relationship between heart rate and physical activities. The notion of intensity refers to the percentage of heart power when performing a physical exercise. Circular takes into consideration the fact that there are variations depending on the subject’s heart abilities, physical condition, age and gender.

The notion of activity intensity is highly correlated to your Maximum Heart Rate (MHR), the fastest rate at which your heart can beat in one minute as it is based on a percentage of your MHR. 

Activity Intensity Circular

Weekly Activity Intensity Circular

Circular divides intensity into three levels:

  • Low intensity activity → from 30% to 55% of your MHR
  • Medium intensity activity → from 55% to 75% of your MHR 
  • High intensity activity → from 75% to 90% of your MHR

Low intensity activity

This the activity you perform during your daily routine. When you exercise at a low-intensity level, you are able to breathe normally. Under normal conditions, low-intensity exercise will not make you sweat. Examples might include taking an easy walk, stretching, shopping or light gardening. 

Medium intensity activity

With medium intensity level, you breathe harder and deeper than at a sedentary level. After around 10 minutes of exercise, you should break into a sweat. Riding a bike on flat surfaces, brisk walking, jogging, endurance exercises and playing doubles tennis are examples of medium intensity activity.

High intensity activity

At a vigorous level of exercise your heart is working at maximum efficiency. Talking will be difficult at this level and you will breathe rapidly. You will break into a sweat after just a few minutes doing exercises like biking or hiking on hills, swimming laps, playing basketball, boxing or doing high-intensity aerobics like HIIT program.

Some research showed that if you get more than 12 minutes of 85 percent of your maximum heart rate in a workout you’ll keep burning 15% more calories even after you stop working out as afterburning during the next 24 hours.

Knowing how we define physical activity intensity, we also explain How Circular tracks your activity volume.

The Training zone, EPOC and recovery for weight loss: science-backed theory

A HIIT “High Intensity Interval Training” is designed to push you into the 85% of your Max Heart Rate zone and will create EPOC “Excess Post-Exercise Oxygen Consumption”. EPOC is what burns calories even after your workout is over and gives you noticeable lasting results in weight loss. Some research showed that if you get more than 12 minutes of 85 percent of your maximum heart rate you’ll keep burning 15% more calories even after you stop working out as afterburning during the next 24 hours. HIIT is used as a time-efficient program to improve physiological functions, and cardiovascular disease risk factor at the same time.

However, it is important to take some recovery time after a HIIT session to rest your heart and body. You should allow your heart rate to come all the way back down to 55-60 percent.

**Be careful, you must be used to physical activities before you engage into HIIT. You can’t start doing HIIT if you are starting your fitness journey. If you are not trained enough you may have some problems with recovering and you may also damage your heart.

If you are a beginner you can first target the medium intensity zone, about 75-85% of your MHR with a cardio activity.

How can Circular help me with that?

The Circular ring continuously monitors heart rate. With the Circular app you can monitor your heart rate during your workout with the live heart rate features to reach the targeted HR zone.

Then, after a workout you can get a better idea of the intensity reached during this workout. You can find a daily summary of the training zone you have reached, which you can compile weekly and compare with your trends to track your progress. By wearing the circular ring during your training you can quantify the intensity you produce and adapt it to your fitness goal.

On the days where you are supposed to take it easy, closely monitor your heart rate during exercise and make sure it doesn’t go above 75 percent of your Max Heart Rate. For recovery, you can use the Circular HRV and RHR analyses or rely on your Energy score to know if your body and heart have recovered.

Energy Score Circular

recovery Circular

Scientific sources:

Int J Environ Res Public Health. 2018 Jul; 15(7): 1508: The Effects of High-Intensity Interval Training vs. Moderate-Intensity Continuous Training on Heart Rate Variability in Physically Inactive Adults. Abdullah Alansare, Ken Alford, Sukho Lee, Tommie Church, and Hyun Chul Jung