"self-control is a strenuous act"
In one study, college students divided into three groups. One group had to give a speech supporting raising tuition at their college. A second group chose between a speech for or against tuition hikes. A third group proceeded directly to the second stage – those devious, unsolvable puzzles. This time, the no-speech group and the group that gave the speech with which they likely disagreed both lasted about twice as long as the people who got to choose what they spoke about. The results suggested it wasn’t just restraint in the face of desire that could deplete your ego, but any choice at all. The subjects who didn’t have to choose a topic were able to allow their volition to take a break, and their ego energy reserves remained intact.
Another study had participants attempt to show and feel no emotion while watching video of either stand-up comedy or an actor pretending to die from cancer. They then tried to solve word puzzles along with people who watched the same videos with the freedom to feel whatever they wished. This time, the people who exerted emotional restraint subsequently solved fewer puzzles than those who let their feelings flow.
The current understanding of this is that all brain functions require fuel, but the executive functions seem to require the most. Or, if you prefer, the executive branch of the mind has the most expensive operating costs. Studies show that when low on glucose, those executive functions suffer, and the result is a state of mind called ego depletion. That mental state harkens back to the way Freud and his contemporaries saw the psyche, as a battle between dumb primal desires and the contemplative self. The early psychologists would have said when your ego is weak, your id runs amok. We now know it may just be your prefrontal cortex dealing with a lack of glucose.
Remember, no matter what the self-help books say, the research suggests that willpower isn’t a skill. If it was, there would be some consistency from one task to the next. Instead, every time you exert control over the giant system that is you, that control gets weaker. If you hold back laughter in a church or classroom, every subsequent silly notion is that much funnier until you run the risk of bursting into snorts.
http://youarenotsosm.../ego-depletion/