When I camp I have to put my backpack somewhere, may as well be under my head. "naked and alone" TV show is not terribly historically accurate, surely humanoids have always a bag of something or a rolled up something to rest their head on. Alternately my wife can sleep on my arm and with some work to get my arm position right the blood flow is not cut off; theoretically a household of humanoids could sleep in a line, with the unfortunate one on the end being one of those back sleepers. Interesting bonus is if someone wakes up from hearing a tiger approach statistically half the tribe is awakened automatically for free, silently. Interesting problem is one member wants to roll over or use the bathroom at night and everyone is unhappy.