If you sleep on your side, your knees can press together, which angles your hips inwards and pulls them out of a neutral position. A pillow between your knees can help keep your hips aligned, taking a great deal of stress off your pelvis and hip joints. This may help ease tension in your lower back and relieve pain in both the lumbar and pelvic regions. While a body pillow comes with the added benefit of supporting your arms while you sleep on your side, there are also a lot of great knee pillows you could try that will keep your hips lined up without the bulk of a body pillow.