I've noticed that in harem stories, when the antagonist is sleeping (yes, actually sleeping) with his thralls, he usually lies on his back with the women resting their heads on various parts of his anatomy.
In my newspaper's medical section, there was a report that sleeping on your back is bad for you: it encourages parts of the anatomy to flop into a position where they block the airways and cause snoring or sleep apnoea, both of which reduce oxygen to the brain, potentially resulting in impairment.
In an article I read a couple of weeks ago, it claimed that sleeping on your side enables waste products to drain more efficiently from the brain, otherwise they could stick around and clog things up, which could lead to or exacerbate dementia.
So basically, those evil bastards who hoard all the gorgeous women are going to end up as senile, drooling wrecks when they get old.
Bwahahahaha!
AJ