Feeling tired after using a lot of mental power isn’t exactly a new phenomenon, but now scientists can actually explain why it happens. One previous theory suggested fatigue was an illusion created by the brain to make us turn to a more enjoyable activity.
But yesterday, a new study by researchers from Pitié Salpêtrière University Hospital in Paris was published on the issue. Scientists used magnetic resonance spectroscopy (MRS) to monitor brain chemicals over six hours for two groups of people – one group was given complex mental tasks to complete, and the other had easier ones. As expected, the first group showed signs of exhaustion and fatigue, like pupil dilation, feelings of fatigue and loss of self-regulation.
But, the MRS also reported some new findings – higher levels of the amino acid glutamate in the prefrontal cortex of the first group. This chemical in the brain is quite normal, as it’s used by nerve cells to transmit signals to other cells. But, too much of it can also be toxic. The more you think and ponder the activities at hand, the more glutamate builds up in your brain. This glutamate build-up makes other prefrontal cortex activity, like planning how to respond to your boss or thinking strategically, more challenging to do and causes us to feel tired.
“Our findings show that cognitive work results in a true functional alteration – accumulation of noxious substances – so fatigue would indeed be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain functioning,” said one of the study’s authors, neuroscientist Mathias Pessiglione.
“We found that glutamate was accumulating in the region of the brain which controls the tasks we set participants. Our understanding is that the brain has some kind of clearance mechanism to counteract this, which may slow down activity,” said co-author of the study Antonius Wiehler of the Paris Brain Institute.