According to recent research, we have a finite amount of temptation-resisting resources. If we use up all our self-control resisting one temptation, we don't have any left to use against another temptation.

For example, once we've spent our store of self-control on resisting the temptation to over-eat, we're likely to have none left to resist the temptation to just sit and watch TV instead of exercising. And this depletion is corroborated by weakened electrical activity in the part of the brain that lets us know when we're screwing up.

To arrive at their findings, the researchers asked forty college students to suppress their emotions while watching a distressing movie for ten minutes. The idea was to exhaust their self-control ration by having them use it all up to fend off their emotions. The participants were then asked to do another task that required self-control. The more successful a participant was at suppressing emotions, the worse he performed on the second task.

An EEG performed during the second task showed weakened activity in the part of the brain that tells us when we are off-track; that is, it lights up to let us know when we are falling off the wagon and failing to exercise self-control. When those neural systems are pooped out, they can't alert us to a mismatch between our actions and our goals.

So there we sit in a stupor in front of the television, having virtuously resisted eating all day, with nothing going on in the part of the brain that is supposed to tell us that we're blowing our exercise regimen.

Source: Psychological Science, October 2007