Heart rate variability: figuring out what is what

Rohan Mathew

The novel heart rate variability measurement is slowly but steadily taking its ground (and market share), so here we are carefully answering both the trivial questions (“What is good HRV)?” and more advanced ones (“What is SDNN?”)

Our heart does not work at all like clockwork or a metronome. When we assume that the heart rate is 60 beats per minute, this does not mean at all that the heartbeats every second. Contrarily, in reality, 0.87 seconds may elapse between the first and second hits, 1.13 seconds between the second and third, and so on. 

Therefore, heart rate variability (HRV) is a measure of how much the time between two consecutive heartbeats varies in milliseconds. In the heart rate variability chart, If the intervals between heartbeats are pretty constant (for example, the heart beats steadily exactly one time per second), the heart rate variation is considered low. If the duration of such intervals varies greatly (for example, 1.5 seconds, then half a second), this is high variability. 

What can heart rate variability tell us?

In simple terms, variability shows the general state of our body (both physically and emotionally). Also, HRV provides essential information about the effects of the nervous system on the cardiovascular system. Consequently, low heart rate variability may even indicate various pathologies. 

To our dear readers, I want to recommend a detailed and understandable article thoroughly explaining what HRV is, – here  – healthy heart rate variability and why and how exactly HRV reflects the physical and emotional state of the body. From this article, I will be using excerpts to describe what is a good heart rate variability and what causes heart rate variability. To make matters better, the specialists at Welltory wrote the article – a leading free heart rate app, so they also clarify what an HRV score is and what a good HRV score is in their software. 

What causes heart rate variability? 

A change in heart rate is a reaction to any stimulus (both external and internal). Thus, the rapid adaptation of the pulse (as well as its return to a state of rest) points to the good work of both the nervous and the cardiovascular systems. 

A small lesson of anatomy

To better understand the influence upon HRV, you need to know something about our nervous system, namely that it is divided into two main parts: central and peripheral. Moreover, most of the nerve cells (neurons) belong to the peripheral nervous system. 

The peripheral system, in turn, is divided into two more: somatic and autonomic. The first is completely controlled by our mind, will, and consciousness. So, for example, if you want to raise your hand, the somatic nervous system will make sure that everything goes smoothly. Because you have been touched – the somatic nervous system will send a corresponding impulse to the brain.

But the autonomic nervous system is something that is not controlled by our will, which we cannot consciously handle. It is responsible for a heartbeat, breathing, sweating, control of the work of internal organs, etc. The autonomic nervous system is separated into: sympathetic and parasympathetic (or, if you prefer, gas and brake). 

The root of HRV 

And here we come to the very essence. When stress occurs, the brain activates one part of the system, the sympathetic, and turns off the other, the parasympathetic. When the stress has passed, and all the body’s reserves need to be restored, the brain must do the vice versa. Thus, the sympathetic system directly responds to stress, and the parasympathetic system returns the body to normal. 

These two systems have the opposite effect, and, logically, their constant in-between struggle and influence upon the heart is the direct cause of HRV. When you need it, the heart beats fast, with little to no variation, however, at the cost of increased energy consumption. So if you wanted to know how to improve HRV – the correct answer will be to avoid stress prevalence at all costs (and also getting rid of bad habits, by default).

How is heart rate variability measured?

To measure HRV, you need corresponding hardware – heart rate variability device – and software to do all the computing. 


The related hardware market is pretty comprehensive, ranging from optical devices incorporated into fitness watches to standalone electrical chest heart rate monitors. Additionally, you may even use smartphone cameras to gather the readings with some apps, though you sacrifice quite a bit of accuracy for such convenience. 


There are many HRV calculator apps, mainly offering the same functionality regarding heartbeat diagrams and charts. Ergo, it is rational to choose such an app for yourself primarily based not only upon user-friendliness but which have the most prominent research base. In this field, Welltory comes out on top. 

Regarding the statistical methods

The most common algorithm is SDNN. HRV apps use this “Standard deviation of NN intervals” to represent overall variability and draw physiological conclusions since SDNN presents all the cyclic statistical components. 

What is good Heart Rate Variability? 

Imagine you’ve got the average HRV value of 56 ms for the day. A legitimate question now – is it good or bad? 

The point is, Heart Rate Variability is a parameter that very much depends on a particular person (his age, genes, gender, general physical fitness, and a number of other factors). Therefore, you need to independently determine the point from which you should start when analyzing your HRV. To do this, you need to follow only these simple steps: 

  1. Collect your HRV data continuously for at least one to two weeks to get a general idea.
  2. Every morning, check the value of heart rate variability and assess your general condition (whether you feel vigorous and energetic or tired and exhausted).
  3. Estimate the high average indicator of variability and proceed from this value in the future. 

As a rule of thumb – the longer you record HRV, the more accurately you can interpret and understand your measurements.