Why does the mind tire during intense mental work?

[newjerseyrunner]

I've been doing math all day, and I've noticed in the past hour or so, my abilities have started being extremely limited. I had to ask for help here for a problem than at 9am probably wouldn't challenge me much. Why after several hours of intense work does intelligence temporarily drop so drastically? Does the brain sort of get backed up, or is it as simple as literally being out of fuel?

 

[jedishrfu]

Here's a technical study on it:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3113724/