In the 2019 study, researchers set out to see if over-exercising could exhaust our brain as well as our body (via Science Daily). The study involved more than 30 professional male endurance athletes. Over the course of three weeks, one group of men maintained their current level of training, while another group boosted their training activity by 40%.
Using exercise tests, questionnaire data, behavioral exams, and brain-imaging technology, the study findings revealed a link between overtraining and mental fatigue. This was evidenced by reduced brain activity seen in the lateral prefrontal cortex, an area of the brain that plays a role in decision-making and cognitive control. Further reinforcing this notion were the results of standardized tests, which showed that participants who had increased their physical activity levels throughout the study were more likely to select tasks with a more immediate payoff than those that would require more time and effort.