Males in my family have a tendency to sleep with their arms above their head. This usually culminates in my 4-year-old brother waking up extremely pissed off about both his arms being asleep at the same time. As for me, I'm an advocate for sleeping on the floor. In my experience, this goes hand-in-hand with either sitting with your back straight or avoiding sitting altogether. I'm still young, but I noticed a great improvement in my posture and elimination of my lower back pain once I started doing these things. I sleep a lot like that Kenyan guy in the first set of photos. Another position i'd sleep in is on my stomach with my right knee and arm pointing north, and the left arm and leg staying straight down. I swear it was called the RKO, or RYO, something to that effect, it was supposed to aid digestion. But I haven't been able to find it since the first time I read about it. It even had a nice MS paint stick figure demonstrating the pose.