What is the biochemical reason for mental fatigue? Is it known exactly why the brain needs sleep? What's dropping low / going high when we experience mental fatigue? I can see why low glucose could result in mental fatigue, are other reasons known?

This is not the biochemistry, but the brain regions involved are described in this article about an fMRI study: <

EDIT:

From what I can tell, mental fatigue is attributed to low oxygenation levels. Here's a study that examines the effect of creatine in preventing mental fatigue: <