I was just reasoning from the studies that exerting self control depletes your ability to exert more self control. In the study most often cited there is actually an interesting link to this exact phenomenon, though. The participants were asked to refrain from eating chocolate (although there is no indication that these people were trying to diet for any reason). Afterward they were tested on standard decision-making tasks, at which those who had to resist eating chocolate performed significantly worse than those who didn't. Then, they were given a glucose drink and their performance returned to normal. A person dieting would be both exerting self control to resist eating things and also resisting things that would provide more glucose for the brain to be using.

However, I seriously doubt this is specific to self control. I would imagine performing any action that depletes glucose in the brain would have the same effects. Prioritizing tasks and deciding tasks to delegate are a couple I've read which are supposed to be very energy-hungry (I've been reading 'Your Brain At Work' recently and its mentioned in there).