The Science behind CALM. – CALMness Score

CALMness – Your Neurological State, Simplified

In part two of the science behind CALM. I want to talk about the CALMness score. This is an indicator we invented, to represent the state of your mental readiness. It is representative of the balance between your sympathetic nervous system and parasympathetic nervous system. Higher CALMness means the nervous systems are balanced, while a lower CALMness score means you anxious, stressed, or not well rested. Get ready for a lot of 3 letter acronyms.

We already covered the fact that CALM. gets accurate heart beat data via the ECG signal, and the data can be used to correlation between your heart beat timings and your autonomous nervous system (ANS) which controls your unconscious body functions in the sleep analysis post. While the autonomous nervous system balance is used as one of the many features to classify sleep stage in sleep analysis, it is the most important factor considered in the derivation of CALMness score.

There are two main types of the ANS, the sympathetic and parasympathetic. The sympathetic nervous system (SNS) controls your fight or flight response. The activity of this part of the nervous system is heightened when you are dealing with stress, it will raise your heart rate, dilate your pupils, and circulate adrenaline. The parasympathetic nervous system (PNS) controls the rest and digest activities. This part of the nervous system is more active when the body is needing to rest. Generally speaking, when the SNS is more dominant, you are alert, and when the PNS is more dominant. The two are like the gas and break pedals on a car; both are important, but if both are fighting for dominance, it could be trouble.

So how do we find out how active your ANS is? and whether the SNS or PNS is dominant? for this we turn to Heart rate variation analysis (HRV). HRV is the variation of the time between each heart beat; even when your heart is beating 60 times per minute, each heart beat will not be exactly 1 second apart. The time between one heart beat to the next may be 0.97 seconds, and then 1.02 seconds for the next beat, and then 0.99 seconds for the next beat. This time between beats, is called the R-R interval. This is why an accurate ECG signal is needed to do HRV analysis, as most heart rate monitor only gives you the rolling average heart rate over a period of time, and not the exact time between each and every single beat of your heart. While HRV can be used to analyse many things, and it is used in our real time training monitoring as well, but for the analysis of ANS, we take the R-R interval and look at it in the frequency domain by taking a Fourier Transform. Different regions on the frequency response correspond to the activity of the two parts of the ANS.

Your CALMness score will be high when either system is distinctively active, and low when the two systems are both active and fighting for dominance. Meditation and breathing is the best way to control your ANS, as shown on the two graphs below [1].

Left: FFT of R-R intervals in normal state.   Right: FFT of R-R intervals during meditation

The numerical score is taken with several reference points, 100 being the score of a yoga master meditating, 60 for a typical person working at the office, and 20 for a student who just pulled an all-nighter and has exams in 4 hours.


Even with its advanced analytics, CALM. is not a medical device and should not be used to diagnose nerve disorders. If you suspect something may be wrong with your nervous system, consult a doctor for advice, and get a professional examination.

In the next post, we will look at the real time training monitoring with CALM. and what we are looking at to make sure you are training safely, while pushing yourself to improve.

[1] Phongsuphap, Sukanya, Yongyuth Pongsupap, Pakorn Chandanamattha, and Chidchanok Lursinsap. “Changes in Heart Rate Variability during Concentration Meditation.” International Journal of Cardiology 130, no. 3 (November 28, 2008)