Side-lying with a pillow between the knees has the best evidence: a BMJ Open review found side sleeping generally protective against waking spinal pain and stiffness.
The mechanics: knees slightly drawn up, pillow between them, spine aligned instead of twisted. Back-sleeping is the strong second - pillow under the knees. Stomach is the problem: it flattens the lumbar curve and rotates the neck for hours. Caveat: the study base is thin - a position you can actually sleep in beats a "correct" one you cannot. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
What is the best sleep position for back pain?
Side-lying has the best evidence behind it. A BMJ Open scoping review of sleep posture and spinal symptoms found side lying was generally protective against waking pain and stiffness - while warning the overall study quality was thin. The mechanics explain it: Mayo Clinic's guidance is side-sleeping with legs drawn slightly toward your chest and a pillow between your knees, which keeps hips and spine aligned instead of twisted. Back-sleeping is the strong second: a pillow under the knees maintains the spine's natural curve. Stomach-sleeping is the problem position - it flattens the lumbar curve and forces the neck into hours of rotation; if you cannot quit it, a thin pillow under the hips reduces the sag. Two honest caveats: the research base is small and mostly observational, so no position is medicine - and a position that lets you actually sleep beats a "correct" one you cannot maintain. If you wake with back pain most mornings, change position before you change mattress; if pain comes with leg numbness or bladder changes, that is a doctor, not a pillow.