Can you turn over a new leaf, go on a diet, learn a new language, and get up every morning at dawn to meditate and clean your house? The idea of ego depletion says, "no." And it has physical evidence to back that up.
I hope you wanted to click on this post, because if you didn't want to, you're not going to be able to do one of the things you actually needed to get done today. Psychologists now believe that a person's capacity for self-control (on an everyday level) is finite. After a certain amount of willpower is used up on one difficult task, there's nothing left over to get other things done.
What's interesting is that, in this model, the self-control is shared between multiple different areas. It's not that a scanty breakfast of salad will cause you to gulp down a chocolate muffin during your first coffee break. It's that a scanty breakfast of salad will keep you from concentrating on an important project. Or affect your ability to keep your temper with your coworkers. A mentally difficult task will, by the same token, cause you to give up on your diet in the evening.
The overall concept is called ego depletion, and there have been quite a few studies backing it up. In one study, people were asked to complete some basic puzzles, but some subjects were also asked to keep a four digit number in their head, remembering it for later. They were then put into a room with some healthy food, and some cookies. The people who had had to work to remember the number were more likely to eat the cookies than the other group. Another experiment worked things the other way around. First the subjects were put in a room with either cookies or radishes. Others were put in a room with cookies and radishes — but were told not to eat the cookies. They were then asked to work on a geometric puzzle, which, it turns out, was impossible to solve (scientists are mean like that). The point of the experiment was to see who would give up first. The radish eaters held out for 20 minutes, one minute longer than the cookie eaters. On the other hand, the people who had to resist the cookies gave up after eight minutes (I like to think the scientists gave them a cookie afterwards).
There is some speculation that ego depletion is a purely physical thing. The brain uses a surprising amount of calories; perhaps it tires like any other muscle does. One study had its subjects go from one difficult task to another, with a lemonade break in between. One half of the subjects got lemonade sweetened with glucose, and the other half got lemonade sweetened with a calorie-free sweetener. The ones that got glucose did better on the second test of willpower than the other group.
Then again, if sugar is the solution, why did the radish-eaters last longer on the problem than the cookie-eaters? And why can some people, with enough motivation, force themselves to keep to difficult tasks? If you run out of calories during a physical task, the body can't just rustle up more from nowhere. Some scientists turn away from what they call the "mechanistic" model of ego depletion and look at the process model. Simply put, after a grueling time spent on one activity, priorities often shift. Perhaps it's not as important to you to keep a pleasant face when you've been up since dawn cleaning. Maybe a after a tough day eating rabbit food, you realize it's not that important to have a clean house. Or maybe you're just lazy. Now pass me a cookie.