Are metabolic costs needed to explain cognitive fatigue?

When we exercise our muscles ache, fuel reserves (glucose, ATP) deplete, and the hours which follow are well described by the word “fatigue”. Prolonged thinking, too, comes with costs; while we mightn’t gasp for breath during an exam, everyone is familiar with the feeling of exhaustion after a sustained bout of cognitive effort. In their review, Pessiglione et al. (2025) formalize this intuition with an integrative model that combines motivational and metabolic mechanisms. Their MetaMotiF framework offers a compelling account of why the exertion of cognitive control becomes increasingly difficult over time. We question, however, whether positing changes in neuro-metabolites—whether depleting energy resources or accumulating neurotoxins—are needed to explain cognitive fatigue. First, it is unclear what explanatory benefits metabolic costs provide that computational and motivational theories lack. Second, the empirical support for metabolic costs is minimal, and while such research is in its infancy due to only recent technological advances, we wonder whether incorporating metabolic costs into theory puts the cart before the horse.