Hey! Great question! I know exacrly how you feel. Without getting too indepth, the reason your brain physically feels tired is because it is. For decades, neuroscientists have known that the adult brain had a limited capacity for focused concentration, approximately 45 minutes. Multiple studies on concentration, mental acuity, and overall mental function have found that after about 45 minutes of focused, concentrated effort, the neurons and their supporting microglia are fatigued. At that point, they have run through enough of theie stored fuel (oxyegn, glocose, etc.) that they are no longer able to maintain their peak activity levels. This is why most classes are about 45 minutes long. It is also why most movies, drama, action, or otherwise, will have small bouts of comedic relief every 45 minutes or so - to help give our overstimulated brains a bit of a breather before more heavy plot stuff.
I would be delighted to discuss the in-depth answer with you further. Let's schedule a session!