What is the biochemical reason for mental fatigue?
This is not the biochemistry, but the brain regions involved are described in this article about an fMRI study: http://www.sciencedaily.com/releases/2012/12/121210101630.htm

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: http://jtoomim.org/brain-training/watanabe2001-creatine-reduces-mentalfatigue.pdf