Willpower is a paradoxical thing, according to these researchers.  For one, we can use up our willpower.  For example, if you need to work late all week long, requiring a lot of willpower, then your diet may suffer that week. 

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.

This is commonsensical stuff, for sure, but new studies are backing it up.  The paradoxical thing, though, is that – even while willpower is depletable – it’s also like a muscle and can be extended and strengthened.

In psychological studies, even something as simple as using your nondominant hand to brush your teeth for two weeks can increase willpower capacity. People who stick to an exercise program for two months report reducing their impulsive spending, junk food intake, alcohol use and smoking. They also study more, watch less television and do more housework. Other forms of willpower training, like money-management classes, work as well.

Hooray for self-control.