What Is Heart Rate Variability? How to understand and check it

What Is Heart Rate Variability (HRV) & Why Does It Matter?

Heart Rate Variability means the difference in time between heartbeats (scientifically named R-R intervals ), measured in milliseconds.

Today heart rate variability (HRV) seems to be one of the most popular measurements among people who want to analyze their body’s systems, physiological stress levels, risk of getting sick, and much more.

Initially developed in the 1960s to assess astronaut health in space, the heart rate variability measure has since evolved into a widely-used diagnostic and predictive tool used by a broad range of health professionals across the world.

What is HRV and why aren’t all of us using heart rate variability to stay on top of our game?

The truth is there are still a lot of challenges on the road to making heart rate variation mass market-friendly. Having developed a wellness app based on heart rate variability analysis, Welltory has encountered just about all of them, and we’re happy to share our experience with you.

HRV means the difference in time between heartbeats (scientifically named R-R intervals), measured in milliseconds.

Diagram with heart rate variability (HRV) and heart beats per minute (bpm) readings and intervals

HRV or heart rate variability is not the same as heart rate, which is simply the number of beats per minute.

Heart rate variance can help assess your overall health and the state of your autonomic nervous system (ANS) which regulates most of the processes in your body.

In other words, HRV measurements show how well you are coping with pressure.

Heart rate variability range is widely used to assess what’s going on with a person’s body — there are about 20,000 relevant studies on PubMed. Note that the HRV measurement is not an electrocardiogram (ECG) recording, and it can’t be used as a substitute for an ECG or a medical check-up.

What Causes Heart Rate Variability? 

Your heartbeat varies because your body is always adapting to changes. It responds to anything that happens to you: a jog, a cup of coffee, or even an upsetting post on your newsfeed.

The heart generates impulses and beats on its own. Different regulatory systems modify this beat in your body. Their job is to respond to external stressors by speeding up or slowing down the heart rate to help keep your body’s internal environment stable.

The signals sent by these regulatory systems are always playing tug-of-war with your heartbeats, which is what makes your heart rate variable.

It is generally accepted that your heart rate variability measurement depends on the balance between your regulatory systems (the scientific term is the Autonomic Nervous System (ANS), which encompasses parasympathetic (PNS) and sympathetic (SNS) activity.)

The ANS has two branches: the sympathetic branch (often called “fight or flight”) that motivates us to react and perform, and the parasympathetic branch (“rest and digest”) that allows the body to recover after stressful events (physical or mental stress).

However, Welltory considers that there are many more factors in addition to the SNS/PNS balance which influence heart rate interval durations, such as:

  • Heart automatic performance. The heart can beat under the influence of different impulses that are born inside the heart, even without nervous system participation. These impulses appear in the SA node (the sinoatrial node is a group of cells in the wall of the right atrium of the heart which can produce a spontaneous electrical impulse). And hormones also influence this cardiac pacemaker.
  • Extrasystoles. These can provide an impression of high HRV, which could be understood as a parasympathetic activity, but this is not the case.
  • Sometimes, the heart temporarily doesn’t respond to parasympathetic action, meaning that it is impossible to assess the correct heart rate variability parameters.

So, the effects on HRV are cumulative, stemming from the performance of regulatory systems and other factors.

The critical point here is that it is more important to look at each system (sympathetic and parasympathetic) separately rather than in tandem. This is because, when one system is working powerfully for any reason, it may seem that the second isn’t working, which isn’t the case. And knowing which systems in your body are active can still give us information about how much stress the body is under, and how well it’s coping with pressure.

What is a Normal Heart Rate Variability?

HRV interpretation isn’t a one-size-fits-all science. It varies widely from one individual to the next, and depends on many different factors: time of day, the way your nervous system typically responds to stressful situations, and much more.

So, what is a good HRV and what are heart rate variability normal values? – the question you are probably interested in after understanding how it can help and how you can measure HRV.

Again, it is generally considered normal for a healthy heart to have a high heart variability rate, and low HRV is a sign of sympathetic regulation dominance.

As with most things in life, the concepts of high and low HRV are relative. Heart rate variability has optimal range thresholds.  A High HRV measurement far above average heart rate variability values could be a sign of arrhythmia or over-fatigue and exhaustion. Simultaneously, low heart variability is not always a bad sign: for example, it could happen after an intensive workout which would be absolutely normal. It is also essential to take into account HRV recovery- the HRV’s ability to return to normal levels. Even so, increased heart rate variability helps you to handle stress better, and is generally an indicator of cardiovascular health. And a consistently low HRV could point to a higher risk of heart attack or cardiovascular risk.

So does HRV affect blood pressure? Research shows that HRV is associated with cardiovascular diseases and mortality. Hypertension can mean increased sympathetic activity and decreased parasympathetic activity. Reduced HRV has been reported in hypertension, however, HRV fluctuations don’t affect blood pressure measurements. Instead, they are associated with each other.

HRV interpretation isn’t a one-size-fits-all science. It varies widely from one individual to the next. It also depends on many different factors: time of day, how your nervous system typically responds to stressful situations, and much more.

Welltory possesses the world’s largest collection of heart rate variability measurements.  Drawing on this extensive database of individual health and fitness information, Welltory devised an algorithm that adapts to your nervous system type and your heart rate variability normal range.

This self-learning algorithm automated lab work that previously had to be performed by hand. It was trained on over 2 billion data points, tested against hundreds of clinical assessments, and confirmed by hundreds of thousands of conversations with our users.

So, how to improve and how to increase HRV

In a nutshell you can improve your HRV by behaving more healthily:

  • better sleep;
  • better nutrition and hydration;
  • regular sport and exercise;
  • using breathing patterns and meditation
  • avoiding risk factors such as alcohol and smoking in a consistent manner.

For some, this is a way of life, while for others, it’s a difficult transition. A lot of sports coaches and athletes worldwide use the HRV training approach to improve performance indicators.

But what is more important is understanding how each particular change in your behavior impacts your body system. You can only turn the changes into a new lifestyle when you know that it definitely works.

So, this is exactly what Welltory is doing. It is the ultimate tool to track your performance, analyze behavioral patterns and get specific tips on how to improve your overall health with minimal daily effort.

What’s special about the Welltory approach?

The Welltory app aims to improve people’s lives through day-to-day data-driven recommendations for minor adjustments to a person’s behavior.

To give our users personalized interpretations of heart rate variability, we used data to automate processes previously done by hand.

Welltory’s specific advantage is our extensive database of over 2 million heart rate variability measurements populated with people’s lifestyle and health data (a total of over 2 billion data points).

We used big data technology to test the results against 5,000-plus clinical assessments completed by our users. The result was the world’s first self-learning heart rate variability algorithm that depends on person’s HRV baseline. Our algorithm also takes into account factors such as age,  gender, height, weight and past measurement data.

  1. The app tracks your heartbeat via the phone camera and flash using a method called photoplethysmography, or PPG.  (PPG is a healthcare technique that has been used to track the heartbeat since 1972, and it’s been proven to be effective for heart rate variability (HRV) analysis. Research shows that measurements taken with a smartphone camera are just as accurate as those taken with heart rate monitors. But we ran our own study, in which we showed that Welltory’s PPG-based phone camera measurements are equal to measurements taken with Polar chest straps, which are ECG-accurate.)
  2. Then it analyzes the variation in the intervals between your heartbeats with a method called heart rate variability analysis, or HRV analysis (the scientific term is time-domain analysis).
  3. The app clarifies what the results mean for you personally by taking into account your genetic factors, your past measurements, and a wealth of other data which it can get from your different devices such as Apple Watches or other wearables, fitness trackers, heart-rate straps, or sources like Apple Health, Samsung health etc.).

In short – it is your AI coach to get you data-driven advice to achieve well-being and better health metrics.

How do we interpret your heart rate variability measurement?

When you take a measurement, we use this enormous database to compare your HRV metrics with an individual baseline and explain what they meant in terms of Productivity, Energy and Stress for other people of the same age and gender with similar results. That’s how we calculate your Productivity, Energy and Stress scores.

Our calculations of your Productivity, Energy, and Stress scores are based on our studies. They are not intended to be a substitute for professional medical advice, diagnosis or treatment, and do not guarantee an accurate evaluation of your physical and mental health. These scores should be seen as advanced data-driven analysis that aims to make sense of your HRV metrics and make them easier to understand.


Productivity indicates your current capacity to deal with intellectual tasks of varying complexity — to think and analyze.

  • An increase in your productivity score means that you can take on more ambitious tasks and stay focused for more extended periods of time.
  • If you have average productivity scores and energy levels, it’s better to take on some routine work.
  • When your productivity score is low, it’s best not to burden your brain with complex or unfamiliar tasks — you’ll just end up procrastinating.

Productivity evaluation results with 80% score and a readout suggesting to pursuit creative tasks

Good productivity requires optimal stress and energy levels. High productivity scores are excellent, but it’s okay to feel distracted or too tired to think properly from time to time.


Energy reflects your body’s ability to support processes like metabolism, movement and digestion, as well as to recover properly. Your energy level determines how well you can maintain a stable internal environment, or homeostasis, and perform physical activity. While high energy scores are something to be happy about, keep in mind that it’s natural to be low on power in the evening.

  • Check if you have enough energy to do everything you have planned. If you’re low on energy, your plans to conquer the world may have to wait. On the other hand, if you see high scores, don’t miss the opportunity to take on a challenge.
  • Know in advance if you risk feeling spent soon. Look at your Energy trend to see if your body is using energy, actively recovering, or spending and saving energy at an equal rate.

Energy evaluation results with 100% score and a readout telling you’re about to strike a balance

At times, you may be full of energy, even if you don’t feel that way. Energy is directly related to parasympathetic nervous system activity. Sometimes, when the parasympathetic system is at work, with your body actively recovering and storing energy, your score may seem high compared to how you feel. In this case, you’re likely going to feel a surge of power a little later — give the rest of your body a bit of time to catch up.

If you’re sick but still have high energy scores, your body is doing a decent job maintaining homeostasis.


Stress reflects the level of physical stress and its effect on the body.

Stress level evaluation results with 43% score and a readout stating that stress level is normal

Extremely low or too high stress is usually not a good sign — it means you’re under way too much pressure or way too tired. It’s natural to have such stress levels after something challenging, like a workout. However, it’s essential to make sure your stress levels bounce back to normal soon after.


Resilience is your body’s ability to maintain homeostasis regardless of what may be happening to you. High resilience lowers your risk of getting sick or developing complications after an illness.

Resilience evaluation results with a very high score and a readout saying body’s systems are super resilient

Our algorithms calculate it based on heart rate variability analysis. Resilience has nothing to do with the intensity of any symptoms you may be experiencing. For example, you may feel better with a body temperature of 37–38 °C (99.5–101 °F) than of above 39 °C (102.5 °F), but your resilience may be worse.

Important note: This content is here for informational purposes only and may not be used for medical assessment or diagnostic purposes. If you feel unwell, please consult a medical professional.


Balance reflects how active your nervous system is as a whole and shows the activity of its two parts — the SNS and PSNS — in percentage terms. Activity of each of them may change throughout the day depending several different factors. Many people who practice yoga or meditation like to track how their activities tip the balance. However, it is the sum of their most important activity — a number that’s too low may indicate that you’re exhausted and need some rest.

SNS and PSNS balance evaluation results with a chart and a readout that explain ANS activity


Coherence shows how synchronized your brain, heart, and respiratory system are.

Coherence index evaluation results with a chart and a cheering readout with explanation on coherence levels

When your systems are synced up, you are less irritable, tend to think more rationally and work more effectively, are ready to make crucial decisions and complete complex tasks, and feel balanced in general.

Better coherence may be achieved with breathing practices, such as coherent or deep breathing, as well as positive emotions and thoughts.

HRV Score

The HRV Score assesses heart rate variability based on lnRMSSD — the natural logarithm of RMSSD. RMSSD, in turn, is a key variability metric reflecting parasympathetic nervous system activity.

The higher your HRV Score is, the better off you are. 100% means ideal variability, when the sky’s the limit — whether we’re talking mental or physical activities.

HRV score with 56.1% evaluation results and a readout telling variability looks about average

Welltory also provides more detailed measurement parameters and many key metrics.

The most common HRV analysis (or time-domain) parameters are:

  • Mean RR — the average time between each heartbeat in milliseconds.
  • SDNN (Standard deviation of normal-to-normal R-R-intervals) — this number shows whether or not your variability is within the standard overall range. Higher numbers usually indicate that your body is coping better with stress.
  • rMSSD (Root mean square successive difference) — reflects parasympathetic activity (vagal activity) and respiratory sinus arrhythmia (RSA) and shows whether or not the body has had a chance to recover.
  • MxDMn (Difference between maximum and minimum value) — the difference between max and min RR-interval values. The higher it is, the more active your parasympathetic nervous system is, and the better your body recovers.
  • pNN50 (proportion of NN50) — Proportion of adjacent R-R intervals differing by more than 50 ms. It shows how active the parasympathetic system is relative to the sympathetic nervous system. The higher the value, the more relaxed the body is. If the pNN50 is low, you’re either tired or over-stressed.
  • AMo50 (Mode amplitude) — evaluates the sympathetic activity. The higher the value, the more active the sympathetic system is.

There are also some more sophisticated parameters of frequency-domain analysis, which, for example, Welltory carries out with a 300 beat-long user measurement.

These are spectral analysis components like

  • Total Power — reflects the total power of HF, LF, and VLF waves. This shows how much power the body has, and how well it’s adapting to stress. This value drops when you’re over-stressed or sick.
  • HF (High frequency) — shows the power of high-frequency waves produced by the parasympathetic nervous system. The higher this number is, the more relaxed you are.
  • LF (Low frequency) — shows the power of low-frequency waves produced by the sympathetic nervous system. The higher it is, the more mobilized your body’s systems are.
  • VLF (Very low frequency)— shows the power of very low frequency waves, which show how the heart is affected by hormones and reflexes. If it’s too high, this means the body is exhausted or sick, and the autonomic nervous system isn’t coping well with stress.
  • LF/HF — shows which system is working harder: parasympathetic or sympathetic. A value over 2 means the body’s systems are too mobilized, and a value of less than 1 means you’re relaxed. A value between 1 & 2 means your systems are balanced.
  • HF/LF/VLF (Wave balance) — shows the power of high frequency (HF) waves, low frequency (LF) waves and very low frequency (VLF) waves. This shows which system is currently regulating the heart’s activity. The higher the HF value, the more relaxed the body is. The higher the LF value, the more mobilized the body. If the VLF is more active – the body is exhausted or sick and not coping with stress.

So, as you can see, you can get an idea of what is happening in your body-system on a day-to-day basis by merely taking measurements via your phone camera or watch. You will see how your actions or behavior influence your health, can improve it, and enable you to lead a better life.

Learn more:

Why quantify yourself?

Self-tracking and lifelogging is a great way to learn more about yourself so you can be healthier, happier, and more productive.