Structurally, the torso and pelvis are generally similar in size, so the key is aligning and squaring the head and lower extremity. A good pillow should align your head with your upper body if you’re a side or back sleeper.
Back sleepers have their legs on the mattress, which pretty much aligns torso and pelvis. Adding a pillow under the knees can reduce stress on the low back muscles and joints.
Side sleeping is the most common position, bending at the hips and knees, with unsupported upper leg falling forward to rest on the mattress. The pelvis twists in, but the torso tends to stay square, rotating the spine and creating asymmetric muscle and joint stress. If you sleep on one side all the time, the uneven stress molds muscles and joints to make one side become less comfortable.
If you’re a side sleeper, put a small pillow between your legs to raise your thigh. This squares and supports the pelvis, and alleviates low back stress. Look for a knee pillow that stays in place as you turn during the night. These types of supports align your hips and cushion your knees without the excess bulk of a regular pillow. Whatever knee support you choose, try to find one that will attache to your leg with an adjustable, stretchy strap to keep it in place all night.
Take the time tonight to evaluate your favored sleeping positions and then design a sleep posture environment that fits and supports your body.