What just happened? Willpower depletion, says psychologist Roy Baumeister, Ph.D., whose experiments at Florida State University involve making people resist eating fresh-baked chocolate chip cookies in lieu of radishes, and then asking them to solve puzzles. People who had used up their willpower resisting the treats had less stamina for their next task and made poorer decisions. Baumeister says that when this happens, it's harder to hold your the tongue. You become your sourest, snippiest self.
This might help: Our willpower is restored when we do things that make us happy. Humor helps. Baumeister and his colleagues found that when volunteers laughed along with a comedy show, their willpower rebounded (Yes, science is telling you to watch more 30 Rock.) One strategy is to try to schedule tasks that require willpower at a time when you’re feeling strongest, well-rested and well-nourished. “Eat something healthy,” says Baumeister, “and allow enough time for the food to get into your system: 15-30 minutes.”