Sleeping ... Everywhere!

Is it possible to be so tired, you fall asleep in class and you don't even realize it. This has become my problem! I keep falling asleep in almost every class. I think this is because I leave everyday at 7:30 in the morning and although this isn't early for some, this is pretty early when it is everyday. How can some people go off of 3 to 4 hours of sleep and be as awake as those who sleep 11? I don't understand. I guess this is part of college life!