It fascinates me too! I googled it once and its something to do with some chemical released during some particular phase of sleep which causes some muscle to relax allowing automatic and unsolicited engorgement. I figure its an accident of nature which evolved rapidly and became more or less universal due to its affect on reproductive rates.
DukeGuy, as I am female, I don’t have to have ribs removed to enjoy that wonder in life. My slip was no-doubt my subconscious letting me know I have some extracurricular activities I need to attend to.