John Medina, in his book Brain Rules, introduces data that forms the following graph:

The data shows that student attention level takes a dive, approximately 10 minutes into a lesson. This is a natural occurrence (and I believe the reason YouTube videos were initially limited to <9 minutes), but can be dealt with by a good instructor. The problem is most instructors (particularly at the university level) know next to nothing about the learning process and instructional theory. A good instructor will introduce variation into a lesson every 10 minutes to regain the students' attention (as depicted in the below graph).

This variation can be accomplished in many ways - from inserting a student activity to asking questions or otherwise soliciting involvement from the students to changing the delivery style or mechanism.

The problem is that so many instructors see instructing as a one-way communication. If a student is not contributing to the communication, they lose focus and many literally go to sleep.

There are other triggers for sleeping during lessons, such as eating a large meal immediately before the lesson, poor ventilation in the classroom, and tired students.