I wake up with an aching spine sometimes. I can't tell if it's because i'm sleeping in a weird position or if it's because I need a new matress. I sleep on my side with a pillow between my knees, like they suggest, and a pillow for my head, and a pillow to support my torso. But of course, some of those pillows shift during the night. How often are you supposed to replace a matress, anyway?