system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we feel tired after having to process a lot of information input e.g. a long day of learning new things from lectures?
gpt
We basically have different levels of relaxation or paying attention.  When you pay close attention to things, your body increases its activity because something important is going on and you might have to fight or flee.

Humans did not evolve to sit in lectures.  So either your brain decides it is unimportant and you get bored and sleepy or it is important and your body prepares for action.  In reality you probably cycle through those states throughout a day of lectures.

Being in a heightened state of readiness is exhausting.  So at the end of the day you are tired.