I think that's only when they are hammered. I have a feeling this survey is junk science, though ;-)
I always thought nodding off after boinking was due to the release of endorphins , the body's natural, opiate-like pain killer.
They usually get released for "good behavior" type activities, exercise, sex, etc.
Had a talk with a microbiologist sister-in-law a long time ago about how she got sleepy after breast feeding. She concurred.