That time between Thanksgiving and New Year's Day is traditionally known as "the holidays. " But face it — it's actually "the annual...

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That time between Thanksgiving and New Year’s Day is traditionally known as “the holidays.” But face it — it’s actually “the annual willpower challenge.”

We try mightily to resist festive, high-calorie foods. Struggle not to overspend our gift budgets. Promise ourselves we won’t drink too much at parties. Strive to be nice to annoying relatives.

No wonder it’s exhausting. And new research suggests a biological explanation.

Specifically, your glucose. That’s a brain fuel. Sufficient glucose is necessary for complex tasks.

“And self-control is a very hard task,” said Matthew Gailliot, the Florida State University researcher in psychology who led the study team.

Their work will be published next year in the Journal of Personality and Social Psychology. Among the findings: “A single act of self-control causes glucose to drop below optimal levels, thereby impairing subsequent attempts at self-control.”

It’s just the latest insight in our ceaseless effort to grasp the mysteries of willpower. Even the ancient Greeks pondered what they called akrasia, or “lacking command over oneself.” The subject has intrigued philosophers, psychologists, sociologists, economists, physiologists — even religion scholars. Consider Eve, her willpower and that darned apple.

“We have to have resolve, the will to resist — where does that hope come from?” asked Heidi Ravven, a religious-studies professor at Hamilton College in Clinton, N.Y., who studies the history of free will.

Understanding willpower is universally important.

“I look around and see the most pernicious problems in society as failures of self-control, or not exerting willpower,” everything from unwanted pregnancies to economic instability, said Kathleen Vohs, a psychologist and assistant professor of marketing at the University of Minnesota’s Carlson School of Management.

Evidence suggests a “certain kind of pre-wiring” in humans that helps govern willpower, said Walter Mischel, the Niven Professor of Humane Letters in Psychology at Columbia University.

Mischel’s “marshmallow tests” of the 1960s and ’70s are still considered seminal work. Simply put, preschoolers were offered a choice of having one reward immediately (a marshmallow, a cookie, a pretzel) or waiting awhile for two.

Mischel has tracked the children into adulthood. “We’re still in touch; they’re now 44 years old,” he said. Subjects who were able to wait longer for the reward as youngsters did significantly better academically and socially as teens and adults, compared with children who took the treat immediately.

Mischel said he was “surprised immensely” by the strength of the correlation.

He finds Gailliot’s link between glucose and willpower “a very interesting line of work, but not the whole story.” Mischel believes a complex, ongoing interplay between biological and psychological systems continually assists or hinders self-control.

Vohs tested dieters with bowls of M&Ms. While watching a movie, half the subjects were seated next to the candy, half were 10 feet away. After the film, all were asked to complete a difficult puzzle. Those next to the candy gave up on the puzzle more quickly — as if resisting the M&Ms had sapped their willpower.

So does that mean the continual temptations of this festive season erode our willpower to near zero? Why even bother putting up the good fight?

Or, as Ravven said, “Should we all just say, ‘Too bad, I’m going to eat those five pieces of pumpkin pie’?”

No. Everyone has willpower, and can work to strengthen it.

“Absolutely,” Mischel said. “Unfortunately, there’s not a great deal of work that’s been done on how to do that. If I were starting my career again, that’s the work I’d be doing.”

But willpower experts do have a few tips.

“Thinking that you don’t have any willpower around a certain stimulus means you won’t exert any,” Vohs said.

Instead, imagine yourself succeeding. “Think you can do it. Have a very positive expectation,” and that increases the chance you’ll be able to resist, she said.

Once you resist a temptation, “that gives you a sense of control,” Mischel said — which, in turn, bolsters willpower.

“Everyone,” he said, “has the potential to have more willpower than they do now.”