It's no secret that our physical strength and stamina have their limits, but now social psychologists are beginning to look at mental fortitude as finite, too. According to the theory behind a recently coined phenomenon known as "decision fatigue," humans have a limited amount of willpower for making smart choices, and that willpower is depleted throughout the day. Here, a brief guide:

What is decision fatigue exactly?
As a person makes decisions throughout the day, the brain depletes its limited amount of mental stamina, and starts employing one of two shortcuts: Make a rash decision or avoid the act of deciding altogether. That "helps explain why ordinarily sensible people get angry at colleagues and families, splurge on clothes, buy junk food at the supermarket and can't resist the dealer's offer to rustproof their new car," says John Tierney in The New York Times.

Hold on. Is this any different than just being physically exhausted?
Yes. And you might not even be aware that you're low on mental energy. In one experiment, researchers asked shoppers at a suburban mall to try and solve some simple arithmetic problems. Those who had already made a number of shopping decisions gave up the quickest on the math problems. "When you shop till you drop, your willpower drops, too," says Tierney.

OK. But does this matter outside of shopping?
Yes. Researchers say decision fatigue could factor into the cycle of poverty. Throughout the day, poorer folks have to make more decisions and tradeoffs than do affluent people who are less obliged to save money. That can mentally exhaust the poor, robbing them of the willpower to work harder, eat healthier, stop smoking, or study for the big test. Decision fatigue has also raised questions about the wisdom of 12-hour shifts for ER doctors and long work days for judges.

What does decision fatigue mean for dieters?
Plenty. Low glucose levels leave the brain focused more on immediate rewards and less on long-term goals, so dieters get stuck in a "nutritional catch-22": A dieter needs willpower to keep from eating, while at the same time, a dieter needs to eat (thereby consuming glucose) to have willpower. In January, Dartmouth researcher Todd Heatherton announced that a dose of glucose totally reversed the effects of decision fatigue on the brain.

Is there any way to manage willpower?
Good decision making is not an innate human trait, says social psychologist Roy F. Baumeister. "It's a state that fluctuates." Anyone is prone to making poor decisions if he has a low glucose level or is mentally fatigued. It's wise to establish healthy routines that eliminate the need for unnecessary deliberation, and make important decisions earlier in the day and on a full stomach.

Sources: Chicago Reader, New York Times, PsychCentral