When you're mentally tired, your brain sends signals
Finally, there was a way to study mental fatigue. Doctors tried to monitor why multiple sclerosis made people's brains exhausted. What was going on up there? It turns out that the regions that are getting more active when you're mentally tired are the areas in charge of the physical movement.
It was interesting to figure this out, because one might not expect that at all. But it's because your body has to work extra hard to make up for your poor thinking ability to accomplish the same task.