Is "lethargism" a word?

Some days don't you just get TIRED?! I mean deep down exhausted tired. Wow. Why does this happen? I vote for predictable energy levels. Seriously. All day today I've just been ready for a nap. But I've been busy all day, so alas, no nap for me. Perhaps I'm tired because I'm an introvert and have been so "with people" lately? Maybe it's the change in weather (much cooler and darker lately). Maybe it's just the end of the semester and everyone else's lethargic feelings are rubbing off on me. Whatever it is, I'd like to kick it's butt. Because seriously, I'd love to be Christmas baking and cleaning the house and decorating for the holidays and hanging out with friends who will be leaving soon, and all I want to do is wrap myself in a blanket on the couch and sleep. Oh well. Maybe I'll try to have a relaxing evening tonight and a good long sleep. Then we'll see where I'm at tomorrow. Wakefulness and abounding energy have to return sometime, right? Right...? *sheesh*