Why thinking makes you tired
11/10/2026Mental exhaustion is apparently more than just a question of willpower. Researchers led by Alexander Soutschek have shown that brain activity and glutamate metabolism may play a role.
We’ve all been there: your mind is racing, your concentration is waning and your motivation to tackle complicated tasks is dwindling. After prolonged mental exertion, our brain signals that we need a break.
Can this fatigue following mental exertion be explained purely in psychological terms – or do certain neurochemical or biological processes in the brain also change, causing us to run out of mental energy?
Until now, it has been a matter of debate whether there is a biological basis for fatigue following mental exertion. Psychological theories assume that people feel increasingly exhausted when further effort demands more of them than they expect to gain from it. Their willingness to continue working then diminishes, and they prefer to turn to easier or more pleasant tasks.
Neurobiological theories, on the other hand, suggest a link between brain activity, changes in certain neurotransmitters and fatigue. A study by the Paris Brain Institute, for example, provided evidence that prolonged mental exertion alters the concentration of the neurotransmitter glutamate in the prefrontal cortex. This region of the brain is responsible, amongst other things, for regulating attention.
Fatigue: more than just a question of willpower
Alexander Soutschek, Professor of Neuroscience of Decision-Making at the Department of Psychiatry, Psychosomatics and Psychotherapy at Würzburg University Hospital (UKW), and his team wanted to get to the bottom of it: Is activity in the prefrontal cortex actually the cause of mental fatigue, and can increased fatigue be explained by a rise in glutamate concentration?
The results of their study have been published in the journal *Proceedings of the National Academy of Sciences of the United States of America* (PNAS).
Professor Soutschek summarises: “Our study challenges purely psychological explanations for why sustained mental work is perceived as strenuous. It shows that mental fatigue is linked not only to motivational assessments but also to neurobiological limitations. Glutamate, alongside other metabolic by-products, appears to play a role in this. Fatigue is therefore not simply a matter of willpower.”
Implications for fatigue research
Changes in glutamate metabolism could, in future, help us to better understand why mental exertion causes fatigue and which neurobiological processes are involved.
This is not only of interest to people in demanding work environments. Those affected by attention-deficit/hyperactivity disorder (ADHD) also frequently report that they become exhausted quickly following mental exertion. Added to this are conditions such as depression, schizophrenia or chronic fatigue syndrome (ME/CFS), which are associated with persistent exhaustion.
In the long term, a more precise understanding of the biological causes of pathological exhaustion could help to develop new treatment approaches.
Targeted brain stimulation could help
Professor Sebastian Walther, Director of the Department of Psychiatry, Psychosomatics and Psychotherapy: “This important study helps us to understand how metabolic changes in the brain are linked to mental exhaustion and exertion. The findings are groundbreaking for the treatment of many mental health conditions. Targeted brain stimulation techniques are likely to be of great help to those affected in the future.”
Further information on the study design is available on the UKW website.
Publication
Hongmei Xia, Gizem Vural, Antonia Susnjar, Helge J. Zöllner, Rebekka S. Mattes, Alexander Soutschek. A Causal Account of the Neural Origins of Mental Fatigue. PNAS, 2 November 2026, DOI 10.1073/pnas.2534136123
