If you’ve ever ended a long day wondering why you suddenly had zero self-control around the cookie jar, or made six impulse purchases on Amazon, or said something snarky and regrettable to a loved one, you’re not alone. Psychologists have spent decades studying why willpower sometimes feels like it evaporates into thin air. One of the most fascinating ideas in this area of psychology is that self-control works a lot like a muscle that can get stronger with training, but also gets tired when we use it too much. And once willpower is worn out, the brain starts taking shortcuts that can get us into trouble.
A Study with Radishes Suggests Self-Control Has a Cost
The “muscle” metaphor comes partly from social psychologist Roy Baumeister’s early research on ego depletion, which suggested that using self-control in one task can reduce your ability to use it in the next. In a famous study, participants were brought into a room filled with the smell of freshly baked cookies. Some people were allowed to eat the cookies, while others were told they could only eat radishes (which is literally the saddest food swap imaginable).
Afterward, everyone worked on an impossible geometric puzzle. The radish-eaters, whose brains had to exert all that effort to resist the strong temptation, left them with fewer regulatory resources for a subsequent difficult task. The punchline? Self-control is not infinite. When you use it in one place, you may have less available somewhere else.
But Willpower Can Be Strengthened
On the flip side, research suggests that this ‘mental muscle’ can bulk up. For instance, practicing small, deliberate acts of self-control over time can lead to broader gains in self-regulation. In one experiment, participants were asked to maintain good posture for two weeks, use their non-dominant hand for everyday tasks, or follow a specific, effortful study routine. Compared to control groups, those who stuck with these self-control exercises later performed better on unrelated tasks requiring persistence and impulse control (Muraven, 2010).
What’s more, studies suggest that our beliefs about willpower also matter. People who see self-control as a limited resource that easily “runs out” are more likely to show classic ego depletion effects, whereas those who believe willpower is more sustainable or even energizing after use show less depletion across tasks (Job, Dweck, & Walton, 2010).
All in all, it seems the ‘willpower as a muscle’ analogy has some empirical support. And just like how physical exhaustion can lead to an increased risk of injury, so too can mental fatigue lead to less than ideal cognitive outcomes.
When Exhausted, We Slip Into ‘Heuristic’ Thinking
One of the most interesting findings in the self-control world is that when we’re cognitively fatigued, we’re much more likely to rely on habits, shortcuts, and stereotypes (not because we’re bad people, but because those responses require less mental effort).
A study by Bodenhausen (1990) looked at ‘morning people’ and ‘night owls’ and asked them to make judgments about hypothetical individuals. The results were striking. People were significantly more likely to rely on stereotypical judgments of others when they were tired. Morning people stereotyped more at night. Night owls stereotyped more in the morning.
The conclusion wasn’t that people become “more prejudiced” when tired, but instead that stereotyping is an effortless, default cognitive process. When we’re depleted, we gravitate to the mental path of least resistance. And this fits with the muscle model. When willpower is rested, we can override automatic responses. When it’s fatigued, the brain falls back on familiar routines.
Willpower‘s Far-Reaching Consequences
It’s not just about cookies vs. radishes. It’s about the end of a long workday when you snap at someone you love. It’s about making harsher judgments when you haven’t slept. It’s about relying on stereotypes, not from malice, but from mental fatigue.
Willpower can be improved and strengthened, but it can also deplete and exhaustion pushes us toward shortcuts our well-rested selves wouldn’t take. That’s why an effective self-control strategy isn’t just about “trying harder” in the moment but about managing the conditions that support willpower in the long run. In fact, research shows that small, consistent habits, like structuring routines, getting adequate sleep, reducing unnecessary decisions, and practicing brief daily acts of self-regulation, can help build a sturdier foundation for self-control (Oaten & Cheng, 2006). Thus, in some ways, we have control over self-control.
