Scarcity

The brain’s store of willpower is depleted when people control their
thoughts, feelings or impulses, or when they modify their behavior in pursuit of
goals. Psychologist Roy Baumeister and others have found that people who
successfully accomplish one task requiring self-control are less persistent on a
second, seemingly unrelated task.

In one pioneering study, some people were asked to eat radishes while others
received freshly baked chocolate chip cookies before trying to solve an
impossible puzzle. The radish-eaters abandoned the puzzle in eight minutes on
average, working less than half as long as people who got cookies or those who
were excused from eating radishes.

From the NYTimes with some good advice on test taking, dieting and how to increase your will power over time.