Brain drain: Scientists look at why mental exertion causes burnout

It’s a familiar feeling on a Friday evening. After a hard day’s work, you finally agree with friends on where to meet for a night out.

But when you’ve figured out what to wear and where you left your keys, a night on the sofa starts to sound more appealing than one on the tiles.

Now, scientists think so may be able to explain why you feel so tired before you’ve even reached the bus stop: your brain has slowed down to handle the tension.

The brain could suffer something similar to the painful build-up of lactate in the muscles during physical exercise. This could be why the tough mental yards, and resisting the temptation to give up all day long, feel just as heavy.

Prolonged mental activity leads to the build-up of a potentially toxic neurotransmitter in the prefrontal cortex, according to a study published in Current Biology. The researchers suggest that the brain slows down its activity to manage the build-up, offering an explanation for why we feel tired.

“Even when you resist scratching an itch, for example, your brain is exercising cognitive control,” said Antonius Wiehler of the Paris Brain Institute, the study’s first author. Repeated demands on cognitive control functions can lead to fatigue, he said.

The prefrontal cortex is the region of decision-making and cognitive control, which comes into play when the brain overrides an impulse or fights any type of temptation.

The team monitored the brain chemistry of 40 participants as they completed repetitive tasks on a computer. They formed two groups, which performed difficult tasks or easy tasks for more than six hours.

The researchers measured the levels of a neurotransmitter in the prefrontal cortex. They found a greater accumulation of glutamate in the participants who were given the more difficult tasks.

Work that involves a lot of thinking requires the brain to repeatedly resist the temptation to do something less demanding. Not surprisingly, this can make people feel tired, but the brain chemistry behind it is unclear.

Now, researchers suggest that cognitive control may lead to the accumulation of glutamate in the brain, high levels of which can be harmful. because it overexcites the neuronal cells.

“We found that glutamate was accumulating in the region of the brain that controls the tasks we set the participants,” Wiehler said. “Our understanding is that the brain has some sort of clearance mechanism to counteract this, which can slow down the activity.”

The researchers postulate that mental fatigue could be related to the recycling of glutamate that builds up during neuronal activity. “The accumulated glutamate needs to be cleared, which we think is likely to happen during sleep,” Wiehler said.

When the participants were asked to report their level of fatigue, no definitive link was found between glutamate and fatigue: the groups performing difficult and easy tasks reported equal fatigue. The researchers said this could be because fatigue was subjective, and those doing the easy task were not aware of the difficulty of the other.

“The fact that glutamate levels do not track self-reported fatigue is somewhat disappointing, but not surprising because there is often a dissociation between biological characteristics and self-reported fatigue,” said Dr. Anna Kuppuswamy of the ‘Institute of Neurology at University College London. who did not participate in the study.

The researchers only monitored glutamate, but suggest that other related substances might be linked to fatigue. “The study measures a single neurotransmitter in a very specific part of the brain, so we need to look at it more globally,” Kuppuswamy said.

But the results were encouraging, he added. “We know that during physical exercise lactate builds up in the muscles, causing muscle fatigue. It’s kind of intuitive that something similar happens in the brain and this is good first evidence to suggest that.”

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