This comes from a short story (I think) by the author Saul Bellow, I read it a long time ago so I'll have to paraphrase but it's utterly fucking brilliant.
A professor is at some university academic soiree and he goes up to the room of a sagacious old Dean to see him about a thesis he is working on. The old Dean is bored and seems to be nodding off and so the professor, thinking he's being clever, says to the Dean:
"I'm sorry Sir, it appears I'm sending you to sleep."
"On the contrary," replies the Professor "you are keeping me awake."