Your usual sleep position — along with other factors, including your weight and your sex — can strain your back and contribute to development of back pain. Sleeping positions also affect existing back pain, either by letting you sleep comfortably or by making you wake up sore. Similarly, back pain is more likely to keep you awake when your sleeping position provides no relief.
The most common sleeping position is on your side, with your legs and hips aligned and flexed. Because this position leaves your upper leg unsupported, the top knee and thigh tend to slide forward and rest on the mattress, rotating the lower spine. This slight rotation may contribute to back or hip pain. To prevent that problem, place a pillow between your knees and thighs.