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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.

Kickstarter campaign – Update #1

Kickstarter is a very interesting journey!

100% funded in under 4 hours!

200% funded in 24 hours!

0/500 ? No more VIP rewards left!

50/500 ⏳ Now switching to the early bird offer, less than 50 available.

Press coverage:

This week, Circular has been featured in some of the most influential tech outlets following our launch and demos our team had. The Circular team is thrilled to see this excitement and we would like to say thank you!

DIGITAL TRENDS: “Smart rings aren’t a new idea, but this one is arguably one of the best yet”

CORE 77: “Circular is an unobtrusive smart ring that’s easy to wear and forget about. Meanwhile, it will keep track of your activity throughout the day and night and deliver personalized recommendations to you via an accompanying app. “

TRENDHUNTER: “Due to its small and unintrusive size, many consumers prefer its minimal aesthetic to bulkier watch alternatives. The tool can be customized with interchangeable shells for different styles.”

GIZMODO: “It’s a stylish but relatively nondescript ring that could be worn in multiple scenarios without drawing too much attention.”

YANKO DESIGN: “This crystal clear focus and economy of function allows the Circular Smart Ring to be an incredibly sleek, fashion-forward wearable that bridges jewelry and tech. The lack of a screen, or a host of apps, music, images, calendars, clocks, and other ‘unnecessary fluff’ even enables the Ring to have a remarkably better battery life of 48 hours with continuous usage. “

If you haven’t secured your Circular ring yet, you can enjoy the early bird deal before it’s too late: take me to kickstarter campaign!

Tips to increase your Deep sleep

Just like REM sleep, the amount of deep sleep you need varies from person to person. It will depend on what you did during your day, on your age and other factors. For adults, deep sleep is approximately 15 to 20% of total sleep. Elderly people experience a shorter duration.

Here are so top level tips to achieve high quality and restorative deep sleep.

Sleep Cycles Circular

Get to know your sleep habits

The periods of deep sleep are the most abundant and most regenerative at the beginning of your sleep. So the goal is to fall asleep as quick as possible to spend the most time in deep sleep.

There are a lot of trackers such as the Circular Ring that you can use to know your sleep habits, the evolution of your sleep cycle and compare your nights.

Sleep every day at the same time 

By doing so, it will allow you to program your subconscious and prepare your body for falling asleep. As with everything, it will take time for your body to adapt (a few nights for most people). Once calibrated, your body will naturally warn you every night when it is time to go to bed.

Be smart about what you eat and drink

Your nutrition has a big impact on your sleep quality. Minimizing caffeine, alcohol, and nicotine before sleeping seems logical. But less known, avoid having heavy and late dinner. Sugar and carbohydrates reduce quality of deep sleep. Go for a light dinner, without quick sugars and red meat which are the foods that take the longest time to digest.

Do not eat within four to six hours prior to the time you want to sleep.

Be physically active

The more you exercise, the more your body will have to spend time in deep sleep to regenerate. It has been proven that daily exercise allows people to sleep better and stay in deep sleep longer. Do 20 to 30 minutes of exercise a day. However, exercising just before bed could interfere with your schedule. Try to exercise daily at least three hours before you go to bed.

Write your day

You can keep a daily journal by briefly describing before sleeping what happened during your day. When you write your day down, you make your brain lighter in order to enter Deep sleep faster. This will facilitate the work that is done during your REM sleep and leave more room for your deep sleep.

Avoid blue lights

Turn off the TV, smartphone, tablet, computer at least 2 hours before sleep. Or even better, keep all your electronics outside the bedroom. The light that these devices emit stimulate the brain, suppress the production of melatonin (which encourages to fall asleep), and interfere with your body’s biological clock.

Magnesium and Melatonin intake

Recent studies have shown that taking magnesium and melatonin with a dosage appropriate to your body can increase the duration of Deep sleep. See this with your doctor.

How to track how much Deep sleep you are getting?

You can use your Circular ring to know exactly how much Deep sleep you get each night. The app gives you the details of your sleep cycle including the portion of Deep sleep accordingly to your cycles, but also the amount of Deep sleep in percentage and duration of your total sleep. You can also compare your Deep sleep from night to night to monitor your evolution and compare your last night with your own average to understand your sleep.

Sleep Stages Circular

You can also read our article on Sleep better and fall asleep faster. The tips described there also apply to increase deep sleep.

If you wish to improve your REM Sleep you can also read Tips to increase your REM sleep.

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.