Every morning, as soon as he wakes up, Iñaki de la Parra, an endurance athlete from Mexico, uses a heart-rate monitor he straps to his chest and an app on his smartphone to measure the tiny variations in the intervals between his heartbeats.

Like a growing number of athletes, Mr. de la Parra uses these measures of his heart-rate variability, or HRV, to help tailor his workout regimen—with high variability indicating his body is prepared for strenuous exercise and low HRV signaling that his body would benefit from a lighter...