Orthopaedically correct sleep is when you sleep in a position that does not cause stress to the spine, the muscular system, or nerves. The position that works well for most healthy people is the fetal position. A person lies on either side, with the legs flexed toward the abdomen and the back is also slightly flexed. This takes stress off the spine and allows the organs to spread themselves out, so there is less intra-abdominal pressure against the diaphragm. A pillow under the head or neck will deflect a lot of weight from the shoulder you are lying on.

Source:  WebMD