Sleep is supposed to be “recovery time,” but posture can turn it into eight hours of low-level mechanical stress. The spine is a stack of joints, discs, and muscles that prefer a neutral alignment—not perfectly straight, but balanced, without sharp bending or rotation. When a person sleeps with the head tilted up or down, or the low back arched and twisted, muscles may stay slightly active to protect the joints. That can show up as morning stiffness, neck pain, headaches, or low back discomfort that improves after moving around.

Pillows matter because the neck is a smaller structure that is easy to misalign. A systematic review and meta-analysis of pillow trials found that certain designs—especially rubber and spring pillows—were associated with reduced neck pain, less waking pain, and lower neck disability in people with chronic neck pain. Interestingly, that same review reported no clear improvement in sleep quality from pillow changes, suggesting that pillow upgrades can reduce pain without automatically making sleep feel deeper or longer. The review also noted that spinal alignment may be influenced more by pillow shape and height than by simply switching materials.

That “height” point shows up in muscle data. In a comparative study measuring muscle activity (EMG) in side-lying, a mid-height foam pillow (10 cm) was rated most comfortable, while the lowest pillow (5 cm) was least comfortable and was linked with higher activity in a mid–upper back muscle on the side against the mattress. In simple terms, a pillow that is too low can let the head drop, pulling the neck and upper back into a strained position that the body compensates for with muscle effort.

For low back pain, the posture evidence leans the other way: position first, pillow second. A 2025 systematic review reported that supine (on the back) is associated with lower low back pain prevalence because it better supports spinal alignment, while prone (on the stomach) is associated with higher risk due to lumbar strain.

Side-lying is common and can be helpful when alignment is supported; poor alignment (for example, the top leg rotating forward and twisting the pelvis) can worsen symptoms. A scoping review in adults also found early signals that side-lying is often protective, but emphasized that high-quality studies are still limited—so recommendations are best treated as practical risk-reduction, not guaranteed cures.