Hey, it's gonna be fine, I've gone to classes so tired that the lesson went in one ear and out the other. Ask some other students to help you write some notes on a day you're less sleepy or if you've really missed a lot of class then explain the problem to your professor and I'm sure they'll be willing to help you catch up. It's what they're paid for. There's always a way around things. There's no need to freak out.