Why concentrating for long periods makes us tired

Concentrating for long periods of time results in fatigue, which is the brain’s way of diverting us to other more leisurely activities so that it can resume its general functions. — Zuma Press/dpa

Concentrating for a long time usually makes us tired, even though we’re not engaging in physical activity.

Researchers have now found a possible reason for why that could be and how cognitive fatigue can influence decision-making.

During intensive cognitive work over several hours, the neurotransmitter glutamate accumulates in a specific part of the brain, known as the prefrontal cortex, according to the research team led by cognitive neuroscientist Dr Antonius Wiehler from the Pitie-Salpetriere Hospital in Paris, France.

This area is active when you make plans or solve complex problems.

The accumulation of glutamate causes cognitive fatigue, which makes us turn to more gratifying activities, the researchers found in their study published in the Current Biology journal.

According to the team, the results show that cognitive work leads to a functional brain change.

In this process, fatigue is a means to an end, because it causes people to interrupt their work, allowing the brain to continue its general functions.

For their study, the researchers used special procedures that made it possible to monitor the concentrations of specific molecules in the brain several times a day.

They worked with two groups of subjects: one had to solve difficult tasks, while the other was given relatively easy cognitive exercises.

At the end of the day, the demanding cognitive effort resulted in a higher glutamate concentration in the prefrontal cortex than the light cognitive work.

According to neurophysicist Dr Harald Möller from the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, increases in glutamate concentration of several percent have been observed many times under targeted stimulation of various brain regions, e.g. the visual system.

However, a relatively rapid drop occurred when the stimulation ended.

Dr Möller criticised the French researchers’ conclusion that there was a longer-lasting glutamate increase in the intercellular space.

It’s difficult to carry out measurements on this part of the brain, he said.

According to him, the researchers should also have measured the subject’s resting phases, which would have allowed them to assess how the experiment influenced the glutamate increase, as opposed to a previous rest phase. – dpa