Оказывается, сила воли человека это ресурс, причем ограниченный - если потратить ее слишком быстро, то ничего не останется (по крайней мере, пока "запас" не восстановится).

Rough day at work? You won't feel like exercising

A new study, published today in Psychology and Health, reveals that if you use your willpower to do one task, it depletes you of the willpower to do an entirely different task.
"After we used this cognitive task to deplete participants' self-regulatory capacity, they didn't exercise as hard as participants who had not performed the task. The more people "dogged it" after the cognitive task, the more likely they were to skip their exercise sessions over the next 8 weeks. "You only have so much willpower."
Какое замечательное оправдание! "Дорогая, я сегодня утром пробежал рысцой три километра, поэтому заставить себя помыть посуду я уже не смогу! Ученые доказали!"

Концовка, впрочем, оставляет некоторую надежду:

She says that by constantly challenging yourself to resist a piece of chocolate cake, or to force yourself to study an extra half-hour each night, then you can actually increase your self-regulatory capacity.

"Willpower is like a muscle: it needs to be challenged to build itself," she says.