The Trick with the Kink

Have you ever treated yourself to a massage?
During any therapy session in the supine position, your therapist will kindly place a pillow, wedge, or similar object under your knees so that your body isn’t lying completely flat.

This would cause an additional arch in the lower back, combined with increased muscle tension in the lumbar region. If you lie flat on your back, it won’t be long before you bend your knees to reduce the muscle tension in your lower back. At night, however, the body must lie flat on its back because there is usually no corresponding elevation of the thighs in bed.
Even when lying on your side, a slight elevation of the thighs up to the knees makes sense: Since the knees are narrower than the pelvis, the legs pull downward in the side position and tilt the pelvis. This movement also affects the spine, especially in the lower lumbar region, leading to compensatory tension in the corresponding muscles.
Proper support for the legs is essential for a restful sleep on your back and relieves pressure on the back muscles when sleeping on your side.