...because it's SCIENCE!
From a wire service story: researchers can tell what personality you have by what position you sleep in.
Normally such stories are suspicious because of the media's propensity to take some scientific qualifier ("the data indicate that there may be"), remove the qualifier and trumpet the findings everywhere as if they were definitive. Doesn't do much for the media's image and doesn't do much for science.
Still, though, I sleep on my stomach.