Heart Rate Variability is a way to measure the impact of all the various life stresses on our bodies. Because the central nervous system governs the heart rate, precise monitoring and measurement tells us how stressed the body is.
Modern smart phones and heart rate monitors make it possible to measure heart rate variability yourself. There are several apps available. Here is the one I use.
Each morning after measuring, it gives you a readout like this:
Based on this reading I can tell whether to go hard or focus on stimulating the relaxation response that day.