These guidelines are given, because you need extra support when sleeping on your stomach. So, anything with a particularly soft comfort layer is problematic. There is a high chance your spine won't align right and you'll wake up in pain. This means, many mattresses that have thicker memory foam layers (e.g. Tempurpedic) are probably out. But as always, you have to decide on a case by case basis. On the other hand, as a side sleeper, you are prone to suffering from pressure points and shoulder pain is a common complaint if the mattress is too firm. A better pillow can sometimes be helpful, as it can raise your head more and remove some of the strain on your shoulders. But that might not be good enough. In that case, a zone system could be another piece in the puzzle. Make things firmer throughout, but selectively soften the shoulder area. This could take a while to figure out and take some trial and error. You probably won't be able to tell just by trying for a limited amount of time in the store, and even a few nights' sleep might not be enough. Expect having to experiment for a while until you get it right. I suggest a mattress with a modular design, as that makes these adjustments easier. Personally, I would always go back to buying a FloBeds mattress. It works well for my wife (side sleeper with lots of shoulder issues) and me (much heavier back sleeper). But I realize that this brand is expensive, and nobody can guarantee that any given mattress will work for you. In the end it all comes down to you doing your homework. Answer from Grim-Sleeper on reddit.com