White Water Rafting

My question to the class is what is the best way to handle the student who falls asleep during class?  I do not currently teach any didactic courses (the students don't typically fall asleep in clinc because there is too much going on!), but I have noticed this throughout my own education and as I have observed in other instructor's classes.  As a matter of fact, I was observing a lecture this week and two different students were out!  One of them actually moaned out loud!  This particular instructor did not address it, but I do not know if I could overlook it as an instructor.  What do you think is the best way to handle this?  Thanks for the help!!