For the longest time, willpower was seen more as a character trait than a resource. This has changed. Today, willpower is compared to muscles: a limited resource that depletes quickly and needs time to recover. Improvement through training is possible to a certain degree, but takes time and effort. The phenomenon is usually discussed under the term “ego depletion”: “We use the term ego depletion to refer to a temporary reduction in the self’s capacity or willingness to engage in volitional action (including controlling the environment, controlling the self, making choices, and initiating action) caused by prior exercise of volition.” (Baumeister et al., 1998, 1253)

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One of the most interesting findings on ego depletion is the broad variety of things that can have a depleting effect. "Our results suggest that a broad assortment of actions make use of the same resource. Acts of self-control, responsible decision making, and active choice seem to interfere with other such acts that follow soon after. (Baumeister et al., 1998, 1263f)