A friend of mine who is pursuing her doctorate at Carnegie Mellon recently asked if I knew why it was that writing her dissertation makes her tired all out of proportion with the physical exertion of the seemingly simple task of sitting down and putting words on a screen.
I looked into this a long time ago and discovered that the brain is apparently the body's prime consumer of glucose. According to Popular Science, over 75% of the glucose that gets used gets used by the brain to create neurotransmitters. These levels fluctuate, but the harder you think, the more glucose you need to fuel the thinking.
So get out there and write that paper! Do your homework! Feel the burn!