Does willpower actually run out during the day?

The idea is called 'ego depletion' and comes from Baumeister and colleagues in 1998. In their experiments people performed worse on a self-control task if they had just done another one, as if a shared resource had been drained. An early synthesis reported a medium-sized effect (d around 0.62), and the metaphor spread quickly into books and popular science.

Then came the scrutiny. In 2014 Carter and McCullough re-examined the same body of work with methods for detecting publication bias, meaning that positive findings get published while null results are filed away. They found strong signs of exactly that skew, and after correction the effect was no different from zero. Two large preregistered collaborations then tested the phenomenon with high statistical power. In 2016 Hagger and colleagues had 23 laboratories (2,141 participants total) run a standardized protocol; the pooled effect sat near zero. In 2021 Vohs and colleagues repeated the test across 36 laboratories (3,531 participants) and found a non-significant result (d = 0.06); the data favored no effect four times more strongly than the expected one.

This does not mean the day leaves no trace. The Vohs group saw a somewhat larger effect among those reporting more fatigue, and as early as 2010 Job, Dweck and Walton showed that people who believe willpower is limited are the ones who behave as if it runs out, while those who do not hold that belief do not. Fatigue, poor sleep and falling motivation do affect self-control, but the simple image of a tank emptied by every decision is not supported.

The fuel tank is an intuitive image because it matches how it feels: you are tired by evening and give in more easily. The metaphor also explains failures conveniently: you 'ran out' rather than chose badly, which removes the blame. Publication bias built up the evidence: the many small studies that happened to show an effect were published, while null results stayed in the drawer, so the literature looked stronger than it was. And there is a market. Productivity books, 'decision fatigue' rhetoric and drinks that promise to restore willpower all sell on the premise that the resource is limited and can be topped up. Ironically, Job and colleagues show that the belief itself that willpower runs out is part of the effect, so the myth helps fulfill itself.

Don't plan your day around rationing a willpower that doesn't behave like a balance anyway. What carries over time is structure rather than in-the-moment resolve: build habits, make the wanted choice easy and the unwanted one slightly harder, and keep tasks interesting enough that you don't have to force yourself. When self-control does falter, look for the concrete cause: too little sleep, too little food or too much monotony, instead of blaming your character.