Under certain circumstances, rubbing my butt apparently has the power to render the one doing the rubbing sleepy. There are specific circumstances: the rub-er must already be trying to sleep and can't, and there should be a level of horizontality involved.
I haven't really done a scientific double-blind kind of test or anything, but the statistical evidence is showing a definite trend. I probably need a larger sampling from different women to really test the theory.
Note that this is not a thinly veiled attempt to get my butt rubbed.