This is a great insight, but it's slightly off in that the term "willpower" groups together several unlike things, some of which are affected by glucose depletion and some of which are not. Willpower includes both the ability to resist things which are tempting but unwise (ie, impulse control), and to do things which are necessary but unattractive (ie, motivation). The paper deals exclusively with impulse control, which is linked to glucose levels; motivation, on the other hand, is not. In my experience, elevated blood glucose levels provide an excess of the former kind of willpower, but sap the latter.