Studies show that willpower is like a muscle— you can build it by exercise, and it gets tired when you use it too much.
The book, The Power of Habit, has a lot of great material around this idea — particularly chapter 5. People who come home after a draining day that has tested their willpower in big ways are much more likely to watch more TV, eat poorly, and not exercise. To choose to do something better or healthier takes more willpower than they have left to spend.
So what does this mean to you?