Brain energy is often confused with motivation, but these are two distinct phenomena. Brain energy is the actual metabolic energy available to the neurons, in the form of adenosine triphosphate (ATP) molecules. ATP is the "energy currency" of the cell, and is produced primarily by oxidative metabolism of energy from food.

I think you are right but it isn't as obvious as that. See Roy Baumeister and the thesis that will power is due to blood glucose levels.