Not so long ago, my post-work routine looked like this:

After a particularly grueling day, I’d sit on the couch and veg for hours, doing my version of “Netflix and chill,” which meant keeping company with a cold pint of ice cream. I knew the ice cream, and the sitting, were probably a bad idea, but I told myself this was my well-deserved “reward” for working so hard.

Psychological researchers have a name for this phenomenon: It’s called “ego depletion.”

The theory is that willpower is connected to a limited reserve of mental energy, and once you run out of that energy, you’re more likely to lose self-control. This theory would seem to perfectly explain my after-work indulgences.

But new studies suggest that we’ve been thinking about willpow...