Researchers at the Paris Brain Institute scanned brain chemistry in two groups of volunteers across a workday, one doing demanding cognitive tasks and one doing easy ones, interleaving both with simple economic choices.
By the end of the day, the high-demand group showed measurably higher glutamate concentration in the lateral prefrontal cortex, the brain region responsible for cognitive control, compared with both the low-demand group and a reference brain region. The same group also showed smaller pupil responses and leaned toward smaller, faster, easier rewards during decisions.
The pattern suggests fatigue is not purely psychological, cognitive control leaves behind a buildup of a metabolite that the brain needs time to clear, which may be why effortful thinking feels literally harder by the end of the day. The study used magnetic resonance spectroscopy in healthy adults over a single simulated workday, so whether the same buildup explains fatigue over weeks or in clinical burnout still needs testing.
Read the original study
Wiehler et al., Current Biology, 2022 · doi.org/10.1016/j.cub.2022.07.010
Thinking hard leaves a trace behind.
Source
- Wiehler, A., Branzoli, F., Adanyeguh, I., Mochel, F., & Pessiglione, M. (2022). A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions. Current Biology, 32(16), 3564–3575.e5. https://doi.org/10.1016/j.cub.2022.07.010