"For example, they were asked whether they preferred $10 now or $50 in six months," said study author Bastien Blain, a research associate at University College London. "And those who overtrained were more likely to choose the immediate reward, which is interesting."
Brain scans on the athletes also showed decreased activity in the area involved in decision-making during the study.
"Cognitive control in this situation is the capacity to maintain exercise despite things like muscle pain," Blain explained. "And what we found is there is an intellectual component involved in exercising and it has a finite capacity. You cannot use it forever."
The findings could also explain why some elite athletes see their performance decline when they're working their hardest – a phenomenon known as overtraining syndrome.
It's not the first time science has pointed out the problems with overtraining – researchers out of the University of North Carolina found that frequent and high intensity exercise is linked to a lower libido. In the study of 1,077 active men who ran, walked, biked, swam, or lifted, those who said they trained at the lowest intensities were nearly 7 times as likely to report a normal or high libido than those who trained at the highest levels of intensity.