This is a common question with no simple answer. However, the good news is that in general, whatever sleeping position works for you is the best one. Sleep however your body is able to comfortably rest.
But if you are frequently not sleeping well, or waking up stiff and sore, you may need to try to change it up.
Neck pain- If you often wake up with neck pain, you may need to avoid sleeping face-down. On your back or side may be better. Adjusting the height or density of your pillow may help. And make sure to keep your shoulders on the bed, not up on the pillow.
Low back pain- In the case of low back pain, again you may be best to avoid sleeping face-down. But if that is your preference, you can tuck a pillow under your belly to ease the pressure on your back. If you sleep on your back, tucking a pillow under the knees is often helpful. And if you’re on your side, tucking a pillow between the knees can ease low back pressure.
If there are no comfortable positions for you, help may be needed. Manual therapy may be needed to help release tense muscles or stiff joints. Or certain stretching or strengthening exercises may help relax the body.
Certain sleeping positions are recommended in specific situations. During pregnancy, sleeping on the side is recommended to ease pressure on the abdomen. If snoring is an issue, or in the case of sleep apnea, sleeping on the side will usually be recommended to help open the airways. In the case of digestive issues like heartburn or GERD, sleeping on the left side is recommended, due to the way gravity pulls on the stomach in the abdomen.
Anyways, sleep science tells us not to worry about it too much, and just sleep however you comfortably can. Quality sleep is vitally important for the mind and body to recover and recharge, so if you think you are not getting good sleep, get it checked out.