I believe willpower is tied to blood sugar. I have a more difficult time focusing on difficult problems and getting started on projects when my blood sugar is low. I think many people incorrectly believe the concept of "blood sugar levels" is something only diabetics have to worry about.

Our brains are using about 1/5 of the calories we eat, ~400 calories per day for most of us. Most people are probably unaware of how their sugar-to-blood ratio naturally varies during the day, in response to eating, exercise, sleep patterns, etc -- but most of us have experienced feeling cranky and tired, having a meal, and feeling much more decisive and energized. This is a good example of how a meal raises one's blood sugar levels.