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, and willpower, also fades with effort. But unlike the mechanism of physical fatigue, the cause of cognitive has been poorly understood.