The prefontal cortex (that section of the brain right behind your forehead) is the part that helps us with things like decision-making and regulating our behavior. Self-control, or willpower, falls under this heading, and thus is taken care of in this part of the brain.
To be effective at controlling our urges and making sound decisions, the prefontal cortex needs to be looked after. That means feeding it with good-quality food so it has enough energy to do its job and getting enough sleep.
Via Pamir Kiciman