Back pain after sleeping flat, and side sleep pain symptoms disappear, the following common causes: First, serious strain on the back muscles lead to internal congestion and edema of the back muscles, significant inflammatory reaction occurs thus causing back muscle pain, flat sleep body weight will cause compression of the back muscles, causing increased back muscle pain symptoms, and side sleep back muscles are not compressed, so Pain symptoms disappear. Second, the back muscles in addition to compression during sleep in the flat position, the internal blood vessels of the muscles will also contract, causing insufficient blood supply to the muscles resulting in muscle tissue ischemia and hypoxia, which will also aggravate the pain symptoms. Side sleep back muscle compression is lifted, blood vessels restore blood supply, so you can avoid the accumulation of local acidic metabolites, can relieve back pain symptoms.