Is this from "Thinking, Fast and Slow"? If so, how do you know it's not actually the break/rest that helps, rather than the food? I don't see any controls in that experiment take a break of equal length but not eating, and others who ate without a break.
Anecdote: I discovered when quitting nicotine that first thing in the morning and after lunch were the times when my willpower was weakest. Your mileage may vary.
Didn't "Thinking, Fast and Slow" mention that by injecting glucose again you got back to the initial state?
It might have, though I don't remember anything like that. I did look, because I disputed the conclusion, but I might have missed it anyway.
Eating isn't the problem. The problem is making decisions throughout the day. It takes willpower to say no and ask tough questions, and your willpower gets drained through the day.
He's describing the opposite effect – people are more likely to say yes in the morning and right after lunch, times when they would arguably have the most willpower.