I guess the subject of sleep is covered under “Posture”because you assume a certain position to sleep. So I will share a few thoughts and mention what comes up most often.
Bottomline, though, is: find what works for you and do it. That applies to both mattresses and pillows. No amount of the ‘right’ whatever is going to make a difference if, in the end, it hurts.
It is worth remembering, and this falls under the importance of sleep as much as anything, that you are anywhere from 1/4 inch to one whole inch taller in the morning when you get up than when you went to bed. This is due to the spinal disc compression that takes place as the day (and gravity) wears on as well as to your posture.
Many experts will talk about a pillow between the knees adding support to the pelvis and relieves low back pain; if it does, do it.
Many will also mention that it’s not a good idea to sleep on your stomach; doing so will force you to turn your head (to breath). During the relaxation that comes with sleep the head will turn even further. Then you will develop a habit of turning your head only to one direction most of the time. Then all kinds of changes in your spine will occur which will lead to imbalance and likely symptoms. In the language of a chiropractor, you will be subluxated.
What most experts will never mention is that sleeping on your side is also a problem, or it will be if you do it for a long time. Eventually, you will have neck, shoulder, or upper back trouble (either because you slept on your side or for some other reason).
When you do develop these kinds of problems, sleeping on your shoulders will aggravate and irritate the condition and prevent recovery, or , at least, delay it.
Sleep on your back. Break the habit of stomach or side sleeping: it will take a few weeks to get used to but you will never go back to your stomach or your side; and yo will be doing yourself a big favor down the road.