Salad or burger? Don't try to decide at the end of a stressful day.

RODALE NEWS, EMMAUS, PA—We all have some aspect of ourselves we want to change, whether it's something minor like better meeting deadlines at work or something major like not spending the mortgage on a new wardrobe. Yet, as the old adage goes, old habits die hard, and when it comes to change, failure is more common than we'd like.

If that sounds familiar, you might take comfort in the new book Willpower: Rediscovering the Greatest Human Strength (Penguin Press, 2011), written by Florida State University social psychologist Roy F. Baumeister, PhD, and New York Times science columnist John Tierney. The two men have spent decades studying, and writing about, human nature and what leads some of us more easily than others to fall prey to temptation, and how that temptation can lead to failed marriages, unhealthy diets, and even major Wall Street financial crises like the one in 2008.

Nowadays, our willpower is being taxed more than ever. "Your body may have dutifully reported to work on time, but your mind can escape at any instant through the click of a mouse or a phone," the authors write. "You can do enough damage in a 10-minute online shopping spree to wreck your budget for the rest of the year." All those temptations put a huge strain on our limited supplies of willpower. In fact, people are tempted in some way every four minutes, Baumeister has found. That’s a total of four waking hours every day. In his studies, he's found that 50 percent of the time, we give in to those temptations.

Why are we so weak? You can blame a lack of self-control, but there's really a lot more at play, Baumeister says. For one, contrary to what you might think, you don't have an unlimited supply of willpower. And a long day of decision making can deplete what little bit we do have. "In the same way that self-control uses willpower, making decisions uses it also," he says. "People often describe it by saying their brain is fried or something like that." So when you spend all day at work making decisions, or exerting a lot of mental energy on writing a report or a presentation, your brain runs out of resources to resist temptation, making it a lot easier to run through the drive-thru to get dinner, rather than expend more mental energy trying to decide what to cook when you get home. Baumeister calls it "decision fatigue," and marketing professionals love to capitalize on it—the candy section at the cash register is hard to pass by after you've made all those decisions up and down the aisles at the supermarket.

There's also a physical component, he says. Your brain feeds off of glucose, the sugar your body creates from a number of foods, and Baumeister's research has found that people whose blood glucose levels have dropped also have a harder time concentrating and making decisions, and they're more irritable and emotional. "When you’re exerting a lot of self-control, you’re using a lot of glucose," he says.

So people on diets, for instance, get slammed from all sides. For one, there's the constant bombardment of marketing messages from junk-food companies, and Baumeister has shown that we're only able to resist about 50 percent of the temptations we face. Two, these people likely make a lot of decisions at work or at home, which depletes their ability to resist temptations at mealtimes. And three, they're cutting back on calories, which deprives their brains of glucose—which they need to boost their willpower.