There are 78 organs in the human body, out of which the five main organs are the heart, brain, kidneys, liver and lungs. Life is impossible without these organs and it would seem logical that they should all get equal importance. However, the heart has ruled our imagination for the longest. We love with all our hearts. We have heartbreaks. Our heart beats faster when we see a loved one. Our heart feels full when happy and empty when sad. In fact, our heartbeats change their rhythm or tempo based on the situation. In scientific terms, this is called heart rate variability (HRV).
Let’s examine how HRV is connected to well-being, what a healthy HRV range is, the significance of this metric and more.
What is HRV?
HRV is quite literally the time interval between consecutive heartbeats in milliseconds. The heart rate on the other hand refers to the number of times your heart beats in a minute. This means that if your heart’s BPM (beats per minute) are 60, your heart is not actually pulsing at one beat per second. In fact, there is a variation in the intervals between the beats and they can differ significantly. For example, at 60 BPM, there may be 0.10 seconds between two beats and then 1.10 seconds between the following two beats. This variation is what HRV entails.
Understanding HRV is the key to understanding your body’s readiness to perform at and maintain high levels of fitness and overall well-being.
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HRV and the autonomic nervous system
Every function of the body is controlled by the nervous system, including the functioning of the heart and the HRV. Our autonomic nervous system (ANS) is a part of the peripheral nervous system that controls involuntary physiological processes, including heart rate, blood pressure, respiration, digestion and sexual arousal.
The ANS also controls the firing of the sinoatrial (SA) node to trigger the start of the cardiac cycle. The SA node, the heart’s organic pacemaker, is one of the dominant parts of the cardiac conduction system, which controls the heart rate. The ANS is capable of swiftly sending a signal to the SA node, which in turn amplifies the heart rate within 3 to 5 seconds.
This quick response is necessary during exercise when the heart has to increase its beating speed to keep up with the body’s increased demand for oxygen.
The ANS consists of two anatomically distinct divisions: sympathetic (activating) and parasympathetic nervous systems (deactivating).
The sympathetic nervous system links the internal organs to the brain through spinal nerves. When these spinal nerves are stimulated, they prepare the body for stress by accelerating the heart rate and blood pressure. The sympathetic nervous system triggers what is often termed the fight-or-flight response.
The parasympathetic nervous system’s nerve fibres are the cranial nerves, primarily the vagus nerve and the lumbar spinal nerves. They are also referred to as the ‘rest and digest’ response. These nerves handle inputs from internal organs and when stimulated, they increase digestive secretions and decrease heartbeat.
HRV arises from these two opposing branches concurrently sending signals to your heart. The sympathetic branch triggers stress hormone production. It also raises the heart’s contraction rate and force (cardiac output) and decreases HRV, which is required during physically or mentally strenuous situations. The parasympathetic branch, on the other hand, reduces the heart rate and increases HRV to reinstate homeostasis after the perceived stress has passed. This normal interaction between the two systems helps the heart to swiftly respond to different physical and mental conditions.
Let’s understand the stress response in detail to comprehend the role of the two systems.
When you experience stress, the amygdala—the part of the brain that facilitates emotional processing—sends a distress signal to the hypothalamus, which then activates the sympathetic nervous system. The adrenal glands pump adrenaline (epinephrine) into your bloodstream. Your heart beats faster than normal, pumping blood to the muscles, heart and other organs. Pulse rate and blood pressure go up and you start to breathe more quickly. Small airways in the lungs expand to take in as much oxygen as possible with every breath. Extra oxygen sent to the brain increases alertness. Sight, hearing and other senses become sharper. At the same time, adrenaline paves the way for the secretion of blood sugar (glucose) and fats from temporary storage locations in the body. These nutrients enter the bloodstream and supply energy to all body parts. All these changes happen in quick succession, making your reflexes quick.
As the initial surge of adrenalin reduces, the hypothalamus activates the second component of the stress response system—the HPA axis. This network comprises the hypothalamus, the pituitary gland and the adrenal glands. It relies on a series of hormonal signals to keep the sympathetic nervous system activated so that cortisol is released. Your body thus stays amped up and on high alert. When the threat passes, the cortisol levels fall. The parasympathetic nervous system then deactivates the stress response.
When your nervous system is balanced, your parasympathetic system will constantly signal your heart to beat slower; and at the same time, your sympathetic system will signal your heart to beat faster. This constant interplay between the two systems causes a fluctuation in your heart rate, namely the HRV.
ANS and the vagus nerve
The ANS controls and regulates the function of various organs, glands and involuntary muscles throughout the body. The vagus nerve is the most far-reaching of the 12 cranial nerves. It is the principal constituent of the parasympathetic nervous system. It is referred to as the information superhighway since it originates in the brain and has pathways to almost every organ in the body.
Vagal tone (vagus nerve activity) highlights the ratio between and parasympathetic and sympathetic signals which reflects how efficiently the vagus nerve is functioning. It can be measured with an electrocardiogram (ECG) to assess heartbeat patterns.
When you inhale, your heart beats faster to circulate oxygenated blood throughout your body and when you exhale, your heart rate slows down. The vagus nerve plays an essential role in regulating the heart’s flexibility to adapt. An increased vagal tone is usually correlated with a lower heart rate and a higher HRV.
HRV in healthy individuals can be increased via vagus nerve stimulation, signifying a move from sympathetic activity to parasympathetic activity. The vagus nerve is charged with steering the heart rate via electrical impulses to specialised muscle tissue in the right atrium, where acetylcholine release slackens the pulse. Measuring HRV may provide vital information about the resilience of your heart and vagus nerve.
How to measure HRV?
Proper HRV analysis necessitates the precise measurement of each heartbeat and the time between beats. There are a few different methods to calculate HRV, although the results may differ from method to method.
ECG or electrocardiography: Specialised wires are attached to your chest to collect electrical signals generated by your heart. The electrical activity produced by heart muscle depolarisations (a negative change in the electric charge), spreads as pulsating electrical waves towards the skin. Even though the amount of electricity is quite minute, it can be picked up accurately with ECG electrodes attached to the skin.
Apps and smartwatch systems: Today there are many smartwatches and apps that help you check your HRV. The majority of these widely available, wearable devices use photoplethysmography (PPG) to detect the heartbeat optically by measuring the wave of blood flow. To put it simply, during a full cardiac cycle (start of one heartbeat to the start of the next heartbeat) the arterial blood pressure rises during active ventricular contraction and falls at the time of ventricular filling. Hence the two kinds of blood pressure: diastolic during relaxation and systolic for contraction. The peripheral blood flow can be measured with optical sensors strapped to the ear lobe, wrist or other capillary tissue.
Chest strap heart monitors: This contraption consists of a long elastic band that can be wrapped around your chest. There is a small electrode pad that stays pressed against your skin (to measure the electrical activity of your heart) and the attached transmitter monitors your HRV. It is believed that chest strap heart monitors are more accurate than app and watch systems.
Apps, smart wearable devices and chest strap monitors are more convenient methods of tracking HRV. When used correctly, they can provide the wearer useful information pertaining to their physiological condition.
What is an ideal HRV and how can you use it as a personalised metric?
Strictly speaking, there is no specific ideal HRV. This range differs from person to person, as there are many factors involved. What may be considered as an ideal HRV for you may not be true for someone else. A person’s gender, age, genetics, lifestyle choices, fitness level, and even their environment can affect their HRV.
A greater variability between heartbeats or a higher HRV is considered ideal. It reflects your body’s strong capability to endure stress or indicates that you’re recovering well from previously accumulated stress.
High HRV often occurs when the relaxation response (a parasympathetic arm of the ANS) is more active, resulting in decreased time between heartbeats. In this state of relaxation, you have more flexibility and resilience in switching between low and high heart rates. Therefore, high HRV may imply better cardiovascular health and a remarkable ability to cope with stress.
It is observed that HRV reduces with age. People in their mid-twenties usually have an average HRV in the range of 55–105, whereas people in their sixties and seventies have an average HRV in the range of 25–45. Males often tend to have slightly higher HRV on average than females. Athletes and sportspersons, in general, have a greater HRV than non-athletes, although there are many people who are in great shape and yet have an HRV below the ideal number. This all points to the fact that HRV is highly individualistic.
Once you are aware of your ideal HRV, you can use it as your personalised metric to track your physical and mental fitness levels. You can track your sleep, understand if your diet is working for you and know if your body is ready to push itself in the gym or needs muscle recovery time. A drop in HRV may mean that your body is expecting increased stress or oncoming illness. Having this information can empower you to make better choices regarding your diet and exercise.
Factors affecting HRV and how you can improve your HRV
Physiological factors: Age, gender and circadian rhythm are the factors that are out of your control and can influence the HRV. HRV should be taken into account in short time measurements extending from a few minutes to a few hours. HRV increases during sleep hours and decreases significantly in the hours just before waking up.
Diseases: Many studies have found that HRV was lower in individuals suffering from certain diseases compared to healthy people. Some of these diseases include:
. Heart diseases
· Lung diseases
· Real diseases (chronic kidney insufficiency)
· Psychiatric conditions (panic attacks, anorexia, epilepsy, anxiety disorder, posttraumatic stress disorders, borderline personality disorder and depression)
Fitness/Training styles: Individuals who live an active lifestyle and have high levels of physical fitness can attain an increase in their basic parasympathetic activity and correspondingly an increase in their HRV. However, intensive sports, competitions and overtraining syndrome may also cause a significant cardiac autonomic imbalance between the two ANS pathways (sympathetic and parasympathetic) and may decrease HRV.
Lifestyle choices: Alcohol causes the HRV to decline and the resting heart rate to rise since it is a central nervous system depressant. Chronic consumption of alcohol can reduce HRV. Some studies have shown that drinking alcohol can lower HRV by an average of 22 milliseconds the following day, and the remaining effects of alcohol in your system may even scale down your HRV for almost 4–5 days. Active as well as passive smoking can increase HRV to unhealthy levels in both men and women.
There are a number of external factors that may influence HRV. These include:
Climatic conditions: While exposure to cold doesn’t affect HRV, heat increases sympathetic nervous system activity, reducing HRV. These changes occur due to the physiological reaction of the autonomic nervous system.
Noise pollution/exposure: Exposure to loud noises or continuous noise leads to a decrease in HRV since noise acts as a stressor.
Pain: Induced pain results in a lowering of HRV.
While some of the above factors may be out of one’s control, there are many ways by which you can improve your HRV. Some are listed below:
Stay hydrated: When you are properly hydrated, your blood can circulate easily and deliver oxygen and essential nutrients to your body. Dehydration may create extra stress and delay recovery which can affect your HRV. Consuming water post-exercise has been shown to increase parasympathetic reactivation.
Regular Healthy Diet: Junk food can trigger an inflammatory response in the body. Low HRV is correlated with increased inflammation. Eating at irregular hours can have the same debilitating effect on your HRV. By eating healthier, you improve HRV, which in turn helps to regulate your sense of hunger and satiety patterns.
Quality of Sleep: It’s not just how many hours of sleep you get, but also the quality and consistency of sleep that matters. Going to bed and waking up at the same time every day is the key to getting a good night’s rest. Consistent sleep helps you have more REM and deep sleep, which improves your circadian rhythm and increases your HRV.
Be exposed to natural light: Go outdoors during the day as often as possible. Exposure to sunlight leads to an increase in parasympathetic activity. Our biological processes that control our sleep-wake times, energy levels and hormone synthesis get activated when you are outside in natural sunlight, especially in the morning hours.
Take a cold shower: Exposing the body to low temperatures for a short duration (cold showers, ice baths, etc.) stimulates the vagus nerve, which triggers the parasympathetic branch of your ANS and controls HRV).
Practice mindfulness: A study showed that participants who engaged in a mindfulness practice for 5 minutes regularly for 10 days had better HRV. Resonant breathing or coherent breathing refers to breathing at a pace of 5 full breaths per minute. Breathing at this rate has been shown to maximise your HRV and reduce stress.
The importance of monitoring HRV
Monitoring your HRV can help you understand your body better. Your HRV can tell you if your body is under stress or relaxed.
Identifying ANS imbalances: The difference between heartbeats is low when your system is in fight-or-flight mode compared to when you are in a relaxed mode, where the difference between the heartbeats is high.
Detecting stress levels: Individuals with low HRV get easily overstressed, while people with high HRV rarely feel stress and their cardiovascular system is in good shape. Checking your HRV can help you learn how to react to stressful situations in a better way.
Reassessing your lifestyle habits: Your HRV can indicate if your lifestyle habits are good or bad for your heart. For example, drinking alcohol can significantly lower your HRV. Knowing your HRV can motivate you to make better lifestyle choices.
Reaction to environment and emotions: Your HRV can tell a lot about how you are reacting to the situation around you. It can detect if you are comfortable in your physical environment or how you are dealing with certain emotions and feelings.
Increase in self-awareness: Tracking HRV can make you more aware of how your thoughts, actions and behaviour can affect your nervous system and impact your bodily functions. For instance, you may realise that staying around negative people is stressing you out and causing you to have lower HRV. So, you can consciously surround yourself with people who lift you up rather than bring you down.
HRV and fitness training
By now you have learned that you can use HRV to your advantage and get into a great physical and mental shape. By tracking your HRV you can understand which days you need to push yourself to the maximum at the gym and when you need to focus on repair work. In this fashion, you can achieve your goals faster and in a much safer and sustainable way. Moreover, it will also ensure that you do not suffer from exercise burnout, which can be physically and mentally taxing. In fact, many elite athletes use HRV to monitor their sleep, exercise and diet, which in turn helps them plan their fitness goals and exercise routines. You can read more about how to use HRV to optimise your fitness training here.
HRV and mental health
Research suggests that HRV is a non-invasive biomarker of mental health resilience. HRV is an index of the body’s flexibility and ability to adapt to stressful situations. When we are in a fight-or-flight mode, the variation between subsequent heartbeats is low. When we are more relaxed, the variation between our heartbeats is high. These reflexes are part of our homeostasis system and at rest, we should be near the middle of the spectrum of sympathetic and parasympathetic activation. A healthy balance means that the body will be able to cope with changing circumstances and stresses in a more efficient way.
Multiple research studies in the last few decades have shown a relationship between low HRV and deteriorating depression or anxieties. HRV has been researched as a biomarker of depression for a long time. According to a research study, patients with major depressive disorder (MDD) had a lower HRV range. Furthermore, post antidepressant treatment over a period of two weeks, these patients showed a normal HRV range parallel to an improvement in symptom severity of depression.
When you find that you have a lower HRV score, it’s time to step back and let your body and mind unwind and relax.
HRV measures the varying time difference between each heartbeat. It also gauges whether there is a balance between the sympathetic and parasympathetic nervous systems. A higher HRV is associated with good cardiovascular health, physical and mental fitness. A lower HRV is associated with physical illness, mental stress, and poor nutrition and sleeping habits. You can improve your HRV by maintaining a good diet, proper exercise regime, fixed sleeping hours and taking care of your mental health.
Now that you have learned the basics of HRV, it’s time to start tracking your HRV. Using this information in your everyday life will help you be the best version of yourself!
The contents of this article are for general information and educational purposes only. It neither provides any medical advice nor intends to substitute professional medical opinion on the treatment, diagnosis, prevention or alleviation of any disease, disorder or disability. Always consult with your doctor or qualified healthcare professional about your health condition and/or concerns and before undertaking a new health care regimen including making any dietary or lifestyle changes.