In a roleplaying game that I'm currently running/playing/somethinging, there's a mechanic built around an attribute called will. Roughly speaking, fighting monsters and making arduous journeys depletes will. Sleeping or relaxing in a safe place replenishes it. Will is also regained after achieving success in some way, which is a more complicated/interesting mechanic.
I think it has interesting consequences for how much attention one pays in lectures. See suppose the characters in the game had to sit through a quantum mechanics lecture (if you're playing the game: ooh, foreshadowing). One character pays attention, takes complete notes and asks questions. She loses a will point for all the effort, but regains it when, at the end of the lecture, she understands operator methods better and can consider her time well spent. Another character disagrees with the lecturer's interpretation of quantum mechanics, but thinks better of starting a philosophical debate in class. Rather than spending a will point stewing over it, she starts doodling in the margins between taking down the most salient points of the lecture. Her understanding of operator methods improves, but she's not satisfied enough to gain a will point. Nonetheless, between taking some notes and worrying about philosophy and doodling, she's lost a will point. She'll be more tired than the character who paid attention. (The character who bunked the lecture is also doing better, but only until exam time.)
Now, if only real life could be modelled by such simple mechanics. Quantisation of willpower, at least, sounds like a viable physical mechanism.
(This picture is mostly just because I like it. It has to do with lectures (phase space representations, not operator methods) and end-of-long-day/weekness. That's all.)