You know those folks that never seem stressed and yet they are getting all their tasks done? Those are the super humans. Inc.com says:
2. Realize that Discipline Runs Out
Studies have shown that you only have so much discipline to give out. If you overuse it, eventually you lose it. Because of this, it’s important to take time to relax and reenergize.
I think by “discipline” they must mean willpower, because studies do show that it runs out. And it can be re-energized by resting it, or using it less for a while, by counting on habit or other ways of acting.