The latest post from Scientific American says that exercising self-control can deplete your brain's decisionmaking muscle.
It turns out that resisting those chocolate-chip cookies or deciding which movie to see exhausts the same muscle that you use for making important decisions--the so-called "executive function". If you tire out your executive function with petty tasks, it won't be able to respond when you call on it to help you make an important decision.
When I was young, I often read Russian folktales about Little Vassilia, whose magic doll would often counsel her when she was distraught and facing a dilemma, "Go to sleep, Little Vassilia, for the morning is wiser than the evening."
Her doll might as well have said, "Go to sleep, Little Vassilia, for you have exhausted your executive function, and eight hours of sleep will recharge you."
P.S. I'm currently in a biggest loser competition, and by exercising extreme restraint in my diet (a typical lunch = carrot sticks, tomatoes, a slab of baked tofu, blueberries, and eight almonds), I've managed to lose 18 pounds in 30 days. By improving my health, am I hurting my decisionmaking ability?
Just in case, any entrepreneurs looking for funding should probably send their executive summaries in before the contest is over!