https://www.outsideonline.com/2302726/how-heart-rate-variability-tells-you-when-hammer

A pretty good overview of the value and application of Heart Rate Variability (HRV).  Although I’m obviously not putting in the training hours comparable to a long-course triathlete, I still believe HRV is a valuable tool for the casual athlete.  The body doesn’t particularly differentiate between the stress it receives from exercise versus numerous other sources such work stress or potential illness. Even if your training volume is relatively low, monitoring your HRV may help recognize when some additional rest and recovery may be in order – potentially warding off injury or sickness.