This happens to me sometimes; back when I was in school I would sometimes find myself fighting to stay awake while driving home in the afternoon/evening on the highway. Some times I wouldn't even recall entire stretches of the trip (muscle memory ftw?!?!). This scared me. I would generally get plenty of sleep beforehand, too. I do think it has a lot to do with just being "bored" of the drive home. Sometimes if I would start to get sleepy, I would turn my music up and sing along loudly. Loud music alone wasn't enough for me, I had to be actively involved with it. As silly as I'm sure I looked to other motorists, it did help