When you prepared your talk you were sure that everyone would be excited to hear it.
"I sat in the back of the classroom, observing and taking careful notes as usual. The class had started at one o'clock. The student sitting in front of me took copious notes until 1:20. Then he just nodded off. The student sat motionless, with eyes shut for about a minute and a half, pen still poised. Then he awoke, and continued his rapid note-taking as if he hadn't missed a beat."
Perhaps you need more than PPT slides and a hoary joke.
"Adult learners can keep tuned in to a lecture for no more than 15 to 20 minutes at a time, and this at the beginning of the class. . .
As the lecture proceeded attention spans became shorter and often fell to three or four minutes towards the end of a standard lecture."
Both of these studies note the severe lapse of attention 15 to 20 minutes into a lecture. Given that students have an attention span of around 15 to 20 minutes and that university classes are scheduled for around 50 or 75 minutes, instructors must do something to control their students' attention. We recommend building a "change-up" into your class to restart the attention clock.
Joan Middendorf and Alan Kalish
Teaching Resources Center