Willpower

I've heard that will power is exhaustable. Hmm.... Could that be true? Supposedly, there's a finite amount of willpower that a person has in any given day. I'm guessing that exercise, nutrition, and sleep affect it, but is it finite? My experience tells me yes....although, with Habitica, I've learned that with the right incentive, I can get much more done in the evenings that I have historically. So, can willpower be replenished during the day? That's an interesting question.