<< How can you stand to sleep for 20 hours straight? >>
You can't. Unless you're in space--since there's no gravity, there's no perception of "laying down" so you could sleep standing up with respect to the bottom of the orbiter (or the Earth) for 20 hours with no problems. Excepty you'll probably groggy and stiff when you wake up.
My gf's horse sleeps standing up, but I don't think he sleeps for 20 hours. In fact, he's so high-strung (ex-racing thoroughbred) I'd bet the guy only sleeps about 3 hours at a time.