An hour of intensive work at the office or studying leaves us physically exhausted more quickly, even though, from a biological point of view, our muscles are still perfectly capable of performing at a high level. Dr Jelle Habay, from the Human Physiology Research Group (MFYS) at the Free University of Brussels (VUB), has studied the complex mechanism underlying mental fatigue and its direct impact on our physical limits. His doctoral thesis demonstrates that the brain is constantly carrying out a subtle cost-benefit analysis that undermines our physical performance, but it also reveals a source of hope: regular physical activity protects us against this mental exhaustion.

The sensation of fatigue is complex. On the one hand, there is physical fatigue, which results from direct physical exertion, such as running or cycling. On the other hand, the MFYS research group focuses on mental fatigue. This form of fatigue is caused by cognitive effort, such as working on a computer or carrying out administrative tasks. Although the origins of these two forms are entirely different — muscles versus the brain — the subjective experience of exhaustion is surprisingly similar.

The chemistry of a tired brain

In living subjects, the brain can only be studied indirectly, which makes investigating the physiological mechanisms particularly difficult. Scientific theories point to changes in the brain’s chemical balance. During prolonged mental activity, certain waste products, such as adenosine, build up. This build-up heightens the perception of the difficulty of physical exertion and impairs the functioning of essential neurotransmitters such as dopamine, which is closely linked to our motivation.

“Mental fatigue leads us to overestimate the effort required to complete the next task, or to see our motivation drop to such an extent that we simply no longer want to make that effort,” explains Dr Jelle Habay. “The brain is constantly carrying out an internal cost-benefit analysis, in which the benefit must always outweigh the effort.”

A psychological barrier with a physiological origin

The most striking finding of this study is the direct link between mental exhaustion and physical performance. When participants are asked to cycle after an hour of intense mental work, they last significantly shorter as a group than when they start the task whilst rested. From a biological and physiological point of view, the muscles are in exactly the same condition; energy reserves are intact and there is no muscle acidification. This limitation is entirely attributable to the brain.

In particular, the researchers hypothesise that the feedback signals sent by the muscles to the brain are interpreted more intensely in a mentally fatigued person, with the result that the same physiological effort is perceived as more strenuous. Ultimately, it is the brain’s conscious decision that determines when we stop a physical exertion.

The impact of lifestyle and everyday stress

Dr Habay also examined significant individual variations: why are some people more sensitive to the effects of mental fatigue than others? The analyses showed that stable factors such as age, gender or general level of mental fitness have, surprisingly, very limited predictive power. Instead, the researchers hypothesise that everyday, dynamic factors – such as sleep quality, acute stress and family circumstances – play a far more significant role.

One stable factor, however, offers clear protection: physical fitness. People who reported engaging in vigorous physical activity each week were found to be significantly better able to mitigate the negative effects of mental fatigue on their physical performance. It appears that their bodies are able to regulate fatigue signals more effectively. Further research will, however, be needed to determine why there is a link between this factor and the response to mental fatigue.

Elite sport and the workplace

These findings have major implications. In elite sport, mental resilience can make all the difference. Techniques such as ‘mental endurance training’ – in which athletes combine cognitive tasks with physical training – are already being studied with a view to pushing the boundaries. Furthermore, understanding acute mental fatigue enables us to take preventive action against chronic conditions such as burnout in the workplace.

Reference

Habay, J., Arauz, Y.L.A., Bian, C. et al. Temporal Robustness of Mental Fatigue Susceptibility: A One-Year Follow-up Study. Sports Med – Open 12, 101 (2026). https://doi.org/10.1186/s40798-026-01062-5

Contact:

Dr Jelle Habay: telephone number available from the editorial team