You hear something a lot about change: People won’t change because they’re too lazy. Well, I’m here to stick up for the lazy people. In fact, I want to argue that what looks like laziness is actually exhaustion. The proof comes from a psychology study that is absolutely fascinating.

So picture this: Students come into a lab. It smells amazing—someone has just baked chocolate-chip cookies. On a table in front of them, there are two bowls. One has the fresh-baked cookies. The other has a bunch of radishes. Some of the students are asked to eat some cookies but no radishes. Others are told to eat radishes but no cookies, and while they sit there, nibbling on rabbit food, the researchers leave the room – which is intended to tempt them and is frankly kind of sadistic. But in the study none of the radish-eaters slipped – they showed admirable self-control. And meanwhile, it probably goes without saying that the people gorging on cookies didn’t experience much temptation.

Then, the two groups are asked to do a second, seemingly unrelated task—basically a kind of logic puzzle where they have to trace out a complicated geometric pattern without raising their pencil. Unbeknownst to them, the puzzle can’t be solved. The scientists are curious how long they’ll persist at a difficult task. So the cookie-eaters try again and again, for an average of 19 minutes, before they give up. But the radish-eaters—they only last an average of 8 minutes. What gives?

The answer may surprise you: They ran out of self-control. Psychologists have discovered that self-control is an exhaustible resource. And I don’t mean self-control only in the sense of turning down cookies or alcohol, I mean a broader sense of self-supervision—any time you’re paying close attention to your actions, like when you’re having a tough conversation or trying to stay focused on a paper you’re writing. This helps to explain why, after a long hard day at the office, we’re more likely to snap at our spouses or have one drink too many—we’ve depleted our self-control.

And here’s why this matters for change: In almost all change situations, you’re substituting new, unfamiliar behaviors for old, comfortable ones, and that burns self-control. Let’s say I present a new morning routine to you that specifies how you’ll shower and brush your teeth. You’ll understand it and you might even agree with my process. But to pull it off, you’ll have to supervise yourself very carefully. Every fiber of your being will want to go back to the old way of doing things. Inevitably, you’ll slip. And if I were uncharitable, I’d see you going back to the old way and I’d say, You’re so lazy. Why can’t you just change?

This brings us back to the point I promised I’d make: That what looks like laziness is often exhaustion. Change wears people out—even well-intentioned people will simply run out of fuel.