What causes our brain to get “tired” and lose focus when thinking hard for extended periods of time like studying or thinking through a difficult concept?

Question

What causes our brain to get “tired” and lose focus when thinking hard for extended periods of time like studying or thinking through a difficult concept?

In: 120

Answer 1

I think glutamate buildup was recently implicated (might have been mentioned in another sub recently) https://www.science.org/content/article/mentally-exhausted-study-blames-buildup-key-chemical-brain