“People who use their willpower seem to run out of it. Smokers who go without a cigarette for twenty-four hours are more likely to binge on ice cream. Drinkers who resist their favorite cocktail become physically weaker on a test of endurance. Perhaps most disturbingly, people who are on a diet are more likely to cheat on their spouse.
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For the last fifteen years, he has been asking people to exert their willpower in the laboratory—turning down cookies, tuning out distractions, holding back their anger, and holding their arms in ice water. In study after study, no matter what task he used, people’s self-control deteriorated over time. A concentration task didn’t just lead to worse attention over time; it depleted physical strength. Controlling emotions didn’t just lead to emotional outbursts; it made people more willing to spend money on something they didn’t need. Resisting tempting sweets didn’t just trigger cravings for chocolate; it prompted procrastination.
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Self-control is an energy-expensive task for the brain [...] Low blood sugar levels turn out to predict a wide range of willpower failures, from giving up on a difficult test to lashing out at others when you’re angry. Gailliot, now a professor at Zirve University in Turkey, has found that people with low blood sugar are also more likely to rely on stereotypes and less likely to donate money to charity or help a stranger. It is as if running low on energy biases us to be the worst versions of ourselves. In contrast, giving participants a sugar boost turns them back into the best versions of themselves: more persistent and less impulsive; more thoughtful and less selfish.”

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