Scientists once offered people cookies, then handed some of them radishes instead

In 1996, researcher Roy Baumeister filled a room with sixty-seven people and the smell of freshly baked chocolate chip cookies. Then came the cruel part. Only some participants were allowed to eat the cookies and chocolates in front of them. The rest were handed a bowl of radishes and told that was their snack, while the treats sat there, in view, smelled, occasionally picked up and sniffed by the disappointed radish eaters.

Afterward, everyone was given a difficult puzzle to solve. The radish group gave up in less than half the time the cookie group did, and tried fewer times before quitting. Baumeister called this ego depletion: the idea that resisting one temptation uses up a limited pool of willpower that then isn't available for the next challenge, even an unrelated one like a puzzle.

A 2010 meta-analysis pulling together eighty-three separate studies on ego depletion found five main factors that drain this reserve: how much effort a task takes, how difficult it seems, how much negative feeling it involves, how tired you subjectively feel, and your blood glucose level. Notice the word subjectively in there. Your sense of your own fatigue, not just the actual state of your body, is enough to burn through willpower before you've done anything.

This is where Stephen Guise's mini habits strategy earns its keep, according to his own reasoning. A single push-up or fifty words of writing requires so little actual effort that there's barely anything to deplete. Because the task is so small, your brain never inflates its perceived difficulty into something intimidating. There's little negative feeling involved, since you're adding something you already want rather than being denied a treat. Your subjective sense of fatigue shrinks along with the size of the ask, because a goal that small feels achievable even when you're genuinely exhausted. And while mini habits can't raise your blood sugar for you, they're the most willpower-efficient way to spend whatever energy you do have.

None of this means willpower stops being a finite resource. It means the size of what you're asking of it determines how fast it runs out, and shrinking the ask is the one lever available to you on the days your reserve is already low.

Before you set any new goal, check it against the five things known to drain willpower: how much effort it takes, how hard it looks, how unpleasant it feels, how tired you already feel, and your blood sugar in the moment. Shrink the task itself rather than trying to push through on a depleted reserve, since a smaller version of the same behavior costs you almost nothing to start. On days you feel worn out, don't skip the habit; make it even smaller instead, small enough that your tired brain doesn't register it as a real demand. Treat your willpower like a limited daily budget, and spend it on starting rather than on forcing yourself through something oversized.