I bought a very strange mattress six months ago. This isn't an advertisement -- I can't strongly recommend it -- but it's called SONU Sleep. It has a "channel" along the top that you can lower your arm into when sleeping on the side. I got it due to persistent shoulder pain from side sleeping. Gradually I've adjusted to it and am now sleeping better.

But that's an argument against my usual paleo heuristic: this sure isn't how we evolved to sleep over millions of years. I wonder if I'd sleep well in orbit.