Some colleges have a mandatory attendance policy. I had a former student who had to retake a math class (which she made a high B in) after missing a week of classes (three classes) due to illness with a doctor's note. Because she had already missed two other classes (one for a funeral, and one because her alarm didn't go off) she failed the class with an 88%. She will have to pay to retake it.
Given the choice of paying to retake a course, and sleeping in class, I'll sleep in class. I will admit to dosing off during lecture classes a time or two in college. A two hour lecture, even about the most interesting topic, will put me right to sleep.
If a school has a mandatory attendance policy, it's presumably because the school believes that the student will learn more by being in class and by listening to the professor or by engaging with classmates. That won't happen if the student is sleeping in class. I maintain my claim that it's rude. If I were the professor, I'd boot sleeping students unless they had some sort of narcolepsy or something. If I taught at a university with an attendance policy, I'd also make sure to keep a record of booting them out of class so that they'd get hit on attendance and fail the course. I don't think there's any real justification for sleeping in class except with some sort of medical condition. Frankly, if the medical condition is so severe that the student can't keep his eyes open for the duration of a class period, the student should consider alternate courses, distance learning, or correspondence school.