Since every body is different, no one position is best for everyone. Believe it or not, when we sleep we actually switch positions around every 20 minutes. This is our bodies way of regulating itself to find the best position for pressure dependent areas in the body such as the spine. If you're someone whose suffering from spine issues, the important thing is to have a firmer mattress; soft mattresses allow for more motion, causing your body to work harder. Another way to get more rejuvenating sleep is to make sure your spine is aligned. Someone with a bigger torso may need a larger pillow than a smaller person. Pillows for other parts of the body can help align the spine as well; If you have back problems you may not want to stretch your legs all the way, a pillow under the knees may be very helpful and side sleepers can put a small pillow between their legs. No one solution will work for everyone and could change from month to month, so its best to try different methods to see which works best for you.