As much as we would love to endlessly engage in intellectually stimulating activities, mental fatigue is a reality. Much like physical tiredness, our brains do get tired when we exert them for long periods of time. However, unlike physical activity, exerting cognitive control is not energy intensive. “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.” So what then causes mental fatigue?
New research by French scientists seem to have found the answer.
“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. That substance, glutamate, is an excitatory neurotransmitter that abounds in the central nervous systems of mammals and plays a role in a multitude of activities, such as learning, memory and the sleep-wake cycle.
In other words, cognitive work results in chemical changes in the brain, which present behaviourally as fatigue. This, therefore, is a signal to stop working in order to restore balance to the brain.”
In a paper, they describe an experiment where they gave two groups of people hard and easy mental tasks and observe the difference in biochemical changes in the brain.
“In particular, they focused on the lateral prefrontal cortex, a region of the brain associated with cognitive control. If their hypothesis was to hold, there would be a measurable chemical difference between the brains of hard- and easy-task participants. And indeed, that is what they found. Their analysis indicated higher concentrations of glutamate in the synapses of a hard-task participant’s lateral prefrontal cortex. Thus showing cognitive fatigue is associated with increased glutamate in the prefrontal cortex. Dr Wiehler speculates that this is the result of a mechanism in the brain that is computing a sort of cost-benefit analysis, with fatigue and increased glutamate adding to the cost of mental effort.
There may well be ways to reduce the glutamate levels, and no doubt some researchers will now be looking at potions that might hack the brain in a way to artificially speed up its recovery from fatigue. Meanwhile, the best solution is the natural one: sleep.”

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