Or get more sleep the night before.
If your instructor doesn't object, bring a light snack (I happen to love oranges when I'm tired and need a boost to keep me going; if you're going to eat them in class though, it would be best to have it already peeled and the wedges separated and put into a plastic container so you're not making a mess or distracted by trying to peel an orange instead of taking notes) or bring a bottle of water or a cup of coffee (beverages might not be a great idea if you have a long lecture though

). I used to have 80 min lectures in college, and my attention span while sitting still seemed to only be about 40 min, no matter how interesting the lecture, so sometimes you just have to do something to snap yourself back, especially if it's that after lunch lecture where you're full and drowsy already).