It’s a familiar feeling on a Friday evening. After finishing a grueling day’s work, you finally agree with friends on where to meet for a night out.
But by the time you have figured out what to wear and where you left your keys, a night on the sofa begins to sound more appealing than one on the tiles.
Now, scientists think they may be able to explain why you feel so weary before you have even reached the bus stop: your brain has slowed down to manage the strain.
The brain could suffer from something similar to the painful buildup of lactate in muscles during physical exercise. This could be why hard mental yards – and resisting the temptation to give up throughout the day – feel equally taxing.
Prolonged mental activity leads to the accumulation of a potentially toxic neurotransmitter in the prefrontal cortex, according to a study published in Current Biology. The researchers suggest the brain slows down its activity to manage the buildup, offering an explanation to why we feel tired.
“Even when you resist scratching an itch, for example, your brain is exerting cognitive control,” said Antonius Wiehler of the Paris Brain Institute, the first author of the study. 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 is applied when the brain overrides an impulse or fights any kind of temptation.
The team monitored the brain chemistry of 40 participants while they completed repetitive tasks on a computer. They formed two groups, who performed either hard tasks or easy tasks for over six hours.
The researchers measured levels of a neurotransmitter in the prefrontal cortex. They found a greater accumulation of glutamate in participants who were given the harder tasks.
Work that involves a lot of thinking requires the brain to repeatedly resist the temptation to do something less demanding. Unsurprisingly, this can leave people feeling tired, but the brain chemistry behind it has remained unclear.
Now, researchers suggest cognitive control may lead to the accumulation of glutamate in the brain – of which high levels can be harmful because it overexcites neural cells.
“We found that glutamate was accumulating in the region of the brain which controls the tasks we set participants,” said Wiehler. “Our understanding is that the brain has some kind of clearance mechanism to counteract this, which may slow down activity.”
The researchers posit that mental fatigue could be linked to recycling the glutamate that builds up during neural activity. “The accumulated glutamate needs to be cleared away, which we think is likely happening during sleep,” said Wiehler.
When participants were asked to report their level of fatigue, no definitive link between glutamate and fatigue was found – the groups performing hard and easy tasks recorded the same tiredness. Researchers said this could be due to fatigue being subjective, and those doing the easy task were unaware of the difficulty of the other.
“The fact that glutamate levels don’t track the reported fatigue is slightly disappointing, but not surprising because there is often a dissociation between biological features and self-reported fatigue,” said Dr Anna Kuppuswamy from the Institute of Neurology at University College London, who was not involved in the study.
The researchers monitored only glutamate but suggest other related substances could be linked to fatigue. “The study measures a single neurotransmitter in a very specific part of the brain, so we have to look at it more globally,” said Kuppuswamy.
But the results were encouraging, she added. “We know that during physical exercise lactate accumulates in the muscles, leading to muscle fatigue. It is kind of intuitive that something similar happens in the brain and this is good first evidence to suggest that.”