The Two Minute Nervous System Test measures the response of your cardiovascular system from supine position to standing position. The main factor considered in this calculation is your heart rate response to the process of standing up.
The increase in heart rate caused by standing indicates how much additional stress this process places on your heart, which is a measure of how high your baseline nervous system stress level is. For example, if there is only a small change in HR as response to standing up, this may indicates that your nervous system is already under significant stress to begin with, as standing up resulted in little increase in heart rate.
Importantly, if you have known heart conditions, or any other chronic ailments that influence your cardiac or nervous system, the expected patterns may not be observed. For instance, conditions like diabetes inhibit the standing HR response, and therefore may make this test of little use to diabetics.
Rthm will remind you to take this test once a week, usually on Sunday, so you can track how your stress levels are fluctuating week to week, and when you have been overdoing things and need to slow down and recover.
To read more about the science, the following is recommended:
Ewing, D., Hume, L., Campbell, I. W., Murray, A., Neilson, J. M., & Clarke, B. F. (1980). Autonomic mechanisms in the initial heart rate response to standing. Journal of Applied Physiology , 809-814.