What is Heart Rate Variability (HRV) And Why is it important?

Technology permeates all aspects of our life providing newer, better, quicker ways to communicate, to learn and to live in comfort. Technology has made our lives easy. It has improved in all spheres of our lives, infrastructures, and industries. Among all industries, healthcare industry is one of the biggest industries which has been affected by this improved technology. It enabled us to check our health status at home without having to visit the hospital. We can now check our vitality signs using wearable devices in daily life from fitness to medical usage; number of steps, burnt calories, Heart rate, blood pressure, body composition and blood sugar.

The Heart Rate Variability (HRV) is growing huge interest. As a matter of the facts, it is rapidly gaining popularity in public, and its studies are being covered by a lot of researchers. HRV is periodic changes in heart rate over time. It is an evaluation tool for tracking the mechanisms of homeostasis of the autonomic nervous system for internal and external environmental factors. For healthy individuals, the heart rate variability changes are large and complex. Its complexity significantly reduced when individual gets threatened by certain disease or stress.

Autonomic nervous system types and functions

Target Parasympathetic Sympathetic


Prepares the body for intense physical activity (fight or flight)

Relaxes the body and inhibits or slows many high energy functions (rest and digest)

Digestive system

Increase peristalsis and amount of secretion by digestive glands

Decrease activity of digestive system


No effect

Causes glucose to be released to blood


Constricts bronchioles

Dilates bronchioles

Urinary bladder/Urethra

Relaxes sphincter

Constricts sphincter


No effects

Decrease urine output


Decrease rate

Increase rate

Blood Vessels

No effect on most blood vessels

Constricts blood vessels in viscera; increase BP

Salivary and Lacrimal glands

Stimulates; increases production of saliva and tears

Inhibits; result in dry mouth and dry eyes

Eye (iris)

Stimulates constrictor muscles; constrict pupils

Stimulate dilator muscle; dilates pupils

Eye (culinary muscles)

Stimulates to increase bulging of lens for close vision

Inhibits; decrease building of lens; prepares for distant vision

Adrenal medulla

No effect

Stimulate medulla cells to secrete epinephrine and norepinephrine

Sweat gland of skin

No effect

Stimulate to produce perspiration

What is Heart Rate Variability?

HRV represents the activity of the autonomic nervous system (ANS) in the body. The activity of autonomic nerves can be analyzed from the microscopic changes between each heartbeat with respect to time. The autonomic nervous system is divided into sympathetic nervous system and parasympathetic nervous system. Activation of the sympathetic nervous system (SNS) increases sweat, heart rate and blood pressure. It decreases bowel movement and dilates pupils dilate. On the other hand, activation of the parasympathetic nervous system (PNS) reduces sweat, heart rate and blood pressure. It increases bowel movements and contracts pupils.

Low HRV generally associated with increased morbidity and stress. It is correlated with stress, illness, overtraining, dehydration, late-night eating, binge drinking, disease, jet lag, excessive exercise, acute and long-term stress. Low HRV tend to appear when individual is in the state of deep sleep, which is third stage in the sleep cycle (awake, shallow sleep, deep sleep, REM sleep).

High HRV normally indicates good health. It is associated with rest, digestion, general health, and recovery. HRV is high when individual is resting, lying down in the cool bedrooms, hiking, doing yoga and meditating. It is also appeared when individual is in REM sleep state which is the last stage in the sleep cycle.

What does it mean?

Let say a man gets into an unpleasant situation, if his HRV increases (not an increase in heart rate, but an increase in rate of change) which means the heart rate is physiologically irregular and complex. It is telling that his body is trying to adapt to the environmental changes. If decreases, it means that the heart rate becomes regular and constant which implies that his body is not in the state of adapting the changes in the surrounding environment.

What would be fun? (What can we interpret?)

If Heart Rate Variability (HRV) allows us to know either the sympathetic nervous system or the parasympathetic nervous system activated, then we can make following predictions based on the possible assumptions.

1) Sleep status

People with dysfunctional sleep ability (e.g., insomnia) observed that their autonomic nervous system function is lower than normal people. This is because, the quality of sleep and stability of the autonomous nervous system are closely correlated together. As the quality of sleep decreases, a tendency of the reducing autonomous nervous system activity can be identified.

2) Physical conditions

HRV reduction means decrease in the complexity of dynamic changes in heart rate, which means decreasing the ability to adapt to ever-changing environments. The mechanism for controlling autonomic nervous system homeostasis can be estimated from small changes in beats that can quickly reach physiological equilibrium by reacting sensitively to blood oxygen concentration, body temperature, and blood pressure.

3) Emotional state

Heart Rate Variability reduction occurs when the pressure on the regulatory system increases. Chronic stress threatens not only the physical health but also the mental health. It may carry such as headaches, insomnia, muscle pain, and palpitations that can be accompanied by mental problems.

A large heart rate fluctuation means that the autonomic nervous system is well regulating, and reduction in HRV is used as an indicator of abnormal and insufficient adaptability of the autonomic nervous system. It is also known that as the age increases, it tends to decline across the board in metabolic syndrome groups, stroke patients and long-term bedridden patients.

What is the purpose of measuring HRV?

– Evaluate non-invasive Autonomic nervous system (ANS) capabilities
– Evaluation of cardiovascular control systems
– Evaluation of the soundness of homeostasis
– Stress management


Age is a strong factor that affects Heart Rate Variability (HRV). Biological age has great impact on homogeneous ability and the body’s agility to stabilize itself in response to the stressor. Low heart rate variability is associated with decreased health, fitness level and youthfulness. It is always important to live the life healthy because biological age often does not match with chronological age. For example, if you are 23-year-old man who does not exercise at all, eats high calorie food every single day and drinks 20 beer per day for the last 10 years, then your chronological age is unlikely sync with your biological age. You would have much higher biological age than your chronological age. Obviously, your HRV value would be much lower than the value that 23-year-old man who regularly exercise and lives healthy life.


Although age have huge influence on HRV, it has been observed that the gender has notable influence on HRV as well. Studies shows that men generally have lower HRV than women in the same age. This implies that men tend to be more sympathetic (fight or flight response) whereas women tend to be more parasympathetic. (rest and digestive response)

How is it measured?

The typically used physiological signals are electrocardiogram (ECG) and photoplethysmography (PPG). Electrocardiogram (ECG) is an electrical signal in fine muscles around the heart activated during a cardiac cycle, and the data are collected by attaching sensors to the chest near the heart or wearing watch/ring type wearable device on the body.

Photoplethysmography (PPG) is a signal that represents a changing arterial wave during each cardiac cycle, and the data are collected by attaching sensors to the chest, wrist, and the finger. Based on the ECG and PPG signals that has been collected using the wearable devices, it extracts accurate features and turns into elaborate HRV value in the way that users can easily interpret.

Bottom Line

Advances in medicine have prolonged human’s life expectancy and increased interest in well-being and healthy life. At the same time, with the development of ICT, an era where individuals can easily manage our health has come. On the hardware side, devices that can easily measure individual health indices are being developed through various sensor technologies. On the software side, the use of big data and advances in artificial intelligence are increasing the accuracy of the analysis and diagnosis of bio signals. Recently, the demand for wearable devices that monitors our health condition in real-time has increased. Detecting stress by measuring physiological signals via wearable devices has become possible in the market.

Current HRV analysis is a non-invasive and reliable test method that can measure autonomic nervous system functions and can be widely applied to various diseases and morbidity related to autonomic nerves. Autonomic nervous system dysfunction or abnormalities are known to be associated with many clinical conditions and symptoms, including fatigue, depression, fibrous myopia, irritable bowel syndrome, poor appetite, dizziness, standing hypotension, diabetes, hypoglycemia, anxiety, asthma, hypertension, arrhythmia, and insomnia. So, to diagnose these symptoms, Identifying the activity or balance of sympathetic and parasympathetic nerves is very important in determining diagnosis, treatment, and prognosis. It could be used as useful information. Another area of useful use through HRV analysis is the determination of the effects of various treatments, exercises, or drugs.