Heart rate variability is such a good index of willpower that you can use it to predict who will resist temptation, and who will give in.
For example, recovering alcoholics whose heart rate variability goes up when they see a drink are more likely to stay sober. Recovering alcoholics who show the opposite response — their heart rate variability drops when they see a drink — have a greater risk of relapse. Studies also show that people with higher heart rate variability are better at ignoring distractions, delaying gratification, and dealing with stressful situations. They are also less likely to give up on difficult tasks, even when they initially fail or receive critical feedback. These findings have led psychologists to call heart rate variability the body’s “reserve” of willpower — a physiological measure of your capacity for self-control. If you have high heart rate variability, you have more willpower available for whenever temptation strikes.
— The Willpower Instinct (Kelly McGonigal), Loc. 613–20