How thinking hard makes the brain tired

livemint.com:

collection for book lists, guest essays and more seasonal distractions. 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. Previous accounts were incomplete. One of the most widely known, the biological one, draws from what is known about muscular fatigue.

It posits that exerting cognitive control uses up energy in the form of glucose. At the end of a day spent intensely cogitating, the brain is metaphorically running on fumes. The problem with this version of events is that the energy cost associated with thinking is minimal.

One analysis of previous studies suggests that cognitively overworked and “depleted" brains use less than one-tenth of a Tic-Tac’s worth of additional glucose. If cognitive fatigue is not caused by a lack of energy, then what explains it? A team of scientists led by Antonius Wiehler of Pitié-Salpêtrière University Hospital, in Paris, looked at things from what is termed a neurometabolic point of view. They hypothesise that cognitive fatigue results from an accumulation of a certain chemical in the region of the brain underpinning control.