This is the greatest mystery of my adult life: How can I spend all day typing at a computer and go home feeling exhausted? How could merely activating the small muscles of my fingers leave me craving the couch at the end of the day? This question actually lies very close to one of the more hotly contested issues in psychology: What causes mental fatigue? Why is desk work so depleting? “It is kind of a mystery, to be honest,” says Michael Inzlicht, a University of Toronto psychologist who studies self-control, motivation, and fatigue.