Sleeping position can affect spinal alignment, acid reflux, sleep apnea, circulation, shoulder and neck pain, and even brain waste clearance during sleep. Side sleeping is often best for posture and reflux, while back sleeping may help lower back pain but can worsen obstructive sleep apnea. Stomach sleeping is usually the least supportive for the neck and spine. The best sleep position depends on your health needs, mattress support, and pillow setup.