From a Wired article by Jonah Lehrer:
…the basic idea behind ego depletion is that self-control and willpower are limited cognitive resources. As a result, when we overexert ourselves in one domain – say, when we’re on a strict diet, or focused on a difficult task for hours at work – we have fewer resources left over to exert self-control in other domains.
Consider this 2007 study: The scientists told subjects to refrain from eating a tempting chocolate donut for a few minutes. Then, they insulted these poor (and probably hungry) experimental volunteers. Not surprisingly, those who had successfully resisted the donut were more likely to get aggressive in response to the insult. Or look at the medical literature, in which people on diets are typically “irritable and aggressive.” (This is the so-called cranky dieter effect.) Although we’d like to be happy and polite, those positive moods take cognitive work, and our brain is too tired to care. We lose our temper because we lack the willpower to swallow our angry words.
Come 3:30 or 4:00, my kids experience myriad variations of “STOP! You must not hop on Pop!” in ever-increasing volume.