How thinking hard makes the brain tired

Physical labour is exhausting. A long run or a hard day’s sweat depletes the body’s energy stores, resulting in a sense of fatigue. Mental labour can also be exhausting. Even resisting that last glistening chocolate-chip cookie after a long day at a consuming desk job is difficult. Cognitive control, the umbrella term encompassing mental exertion, self-control and willpower, also fades with effort. But unlike the mechanism of physical fatigue, the cause of cognitive fatigue has been poorly understood.