This famous radish experiment reveals why willpower runs out

Roy Baumeister, a psychology professor at Florida State University, once asked a group of students to skip a meal and come into his lab hungry. He put them, one at a time, in a room with a plate of radishes and a plate of warm chocolate chip cookies and candy. Some students were told they could eat whatever they liked. Others were told they could only eat the radishes, with the sweets sitting there, smelled but forbidden. Afterward, every student was handed a set of geometry puzzles that were secretly impossible to solve.

The students who'd eaten the cookies worked at the puzzles for about twenty minutes before giving up, roughly the same as a control group that hadn't been offered any food at all. The radish eaters, who had spent several minutes resisting temptation, quit after just eight minutes. As Baumeister and science journalist John Tierney describe it in their book Willpower, resisting the cookies had used up something the radish eaters needed later for a completely unrelated task: persistence on a puzzle.

Baumeister's conclusion is that willpower behaves like a muscle that tires with use, and that we draw on the very same limited reserve for controlling our thoughts, managing our emotions, resisting temptation, performing well at work, and even making ordinary decisions. Every choice you make during the day, he told Vanderkam, quietly spends some of what you'll need later for self-control. That is why, he says, major self-control failures cluster at night: diets are broken in the evening, not the morning, and most impulsive crimes happen after eleven p.m. Mornings, by contrast, come after a night's rest has refilled the tank. One analysis of Twitter feeds even found people worldwide were more likely to use upbeat words like awesome between six and nine in the morning than at other hours. The reserve is highest early, and it only shrinks from there.

Look at your day and pick out the one task that genuinely needs self-discipline, the piece of writing you keep avoiding, the habit you're trying to break, the hard conversation you keep rescheduling. Put that task in the first hours after you wake, while the reserve Baumeister describes is still full, rather than waiting until evening when it's already been spent on traffic, minor irritations, and a hundred small decisions. Try not to load a string of big choices right after it, since deciding also draws on the same resource self-control uses. And when you do slip late in the day, don't read it as weakness, read it as exactly what the radish experiment predicted would happen to anyone by then.