To prevent low back pain when you sleep, use a firm, not necessarily hard, mattress. Sleep on your side with your knees slightly tucked up; this position puts the least strain on your back. A pillow between your knees reduces the pressure even more. If you prefer to sleep on your back, use a pillow under your knees. Sleeping on your stomach will put the spine into an unnatural position and can lead directly to morning backache. Some people use a half-inch sheet of plywood under a sagging mattress to provide better support. Other people use a waterbed, which distributes body weight more evenly than other types of mattresses do. A good rule of thumb is to use whatever is most comfortable for you.