Ever wonder why your resolve to hit the gym weakens after you've slogged through a soul-sapping day at work? It's because willpower isn't just some storybook concept; it's a measurable form of mental energy that runs out as you use it, much like the gas in your car.

Roy Baumeister, a psychologist at Florida State University, calls this "ego depletion," and he proved its existence by sitting students next to a plate of fresh-baked chocolate-chip cookies. Some were allowed to snack away, others ordered to abstain. Afterward, both groups were asked to complete difficult puzzles. The students who'd been forced to resist the cookies had so depleted their reserves of self-control that when faced with this new task, they quickly threw in the towel. The cookie eaters, on the other hand, had conserved their willpower and worked on the puzzles longer.

Further studies have suggested that willpower is fueled by glucose—which helps explain why our determination crumbles when we try to lose weight. When we don't eat, our glucose drops, and our willpower along with it. "We call it the dieter's catch-22: In order to not eat, you need willpower. But in order to have willpower you need to eat," says John Tierney, coauthor with Baumeister of Willpower: Rediscovering the Greatest Human Strength.

But there are ways to wield what scientists know about willpower to our advantage. Since it's a finite resource, don't spread yourself thin: Make one resolution rather than many. And if you manage to stick with it by, say, not smoking for a week, give your willpower a rest by indulging in a nice dinner. Another tactic is to outsource self-control. Get a gym buddy. Use Mint.com to regulate your spending or RescueTime.com to avoid distracting websites. As Tierney explains, "People with the best self-control aren't the ones who use it all day long. They're people who structure their lives so they conserve it." That way, you'll be able to stockpile vast reserves for when you really need it, like hauling your lazy ass to the gym.

Input | Exercise

Output | Performance