We’re not about to crawl between the sheets with you and prescribe stiff rules about how to position your sleeping body. However, if you already suffer from bouts of back pain (whether caused by long hours of office work, heavy lifting, or pregnancy), you might find that sleeping in your regular positions is about as comfortable as having a prickly pear in your pajama bottoms. In this situation, you can try two tricks. First, if you’re a back sleeper, place a pillow under your knees. This takes the strain off your lower back. (If you want to adopt this position permanently, look for a wedgeshaped specialty pillow at a pharmacy or health store.) Second, if you sleep on your side, put a pillow between your knees. Again, this helps keep your spine in the straight, neutral alignment it likes best.
If you’re still not getting any relief for a sore back, consider the pelvic-tilt exercise. Not only can it relieve back pain, but it doesn’t require you to climb out of bed.
Source of Information : Oreilly - Your Body Missing Manual