The Spine's Natural Curves
The human spine has four natural curves: cervical (neck), thoracic (mid-back), lumbar (lower back), and sacral (pelvis). Good sleep posture maintains these curves in neutral alignment — not flattened, not exaggerated. The challenge is that lying horizontal changes the mechanical forces acting on the spine, and different sleep positions load the spine in different ways.
Side Sleeping
Side sleeping is the most common position and generally well-tolerated for spine health — provided pillow height matches shoulder width. The gap between shoulder and mattress can range from 4–7 inches depending on body build. A pillow too thin creates lateral flexion of the cervical spine; too thick creates the opposite. The hips also need lateral support — placing a pillow between the knees reduces pelvic rotation and lumbar stress.
Back Sleeping
Back sleeping allows natural lumbar lordosis to be maintained with the right mattress firmness. A mattress too soft allows the lumbar region to sink and flatten; too firm creates a gap between the lumbar and the surface. A pillow under the knees (not just the head) significantly reduces lumbar muscle tension in back sleepers. Pillow height for back sleepers should be lower than for side sleepers — enough to fill the cervical curve without pushing the chin forward.
Stomach Sleeping
Stomach sleeping is the most problematic position for spine health. It requires rotating the neck to one side (creating sustained cervical rotation) and tends to hyperextend the lumbar spine. Physical therapists generally advise against it for anyone with neck or lower back pain. If you stomach sleep, a thin pillow or no pillow minimizes neck rotation. A pillow under the pelvis (not the abdomen) can reduce lumbar hyperextension.
Mattress Selection by Position
Side sleepers benefit from softer surfaces (3–6 firmness) that allow the shoulder and hip to sink while maintaining spinal alignment. Back sleepers typically do best on medium to medium-firm surfaces (5–7) that maintain lumbar support. Stomach sleepers need firmer surfaces (6–8) to prevent the pelvis from sinking and hyperextending the lower back. Most people switch positions through the night — medium-firm (5–6.5) surfaces work for most combination sleepers.