Lower back pain when sleeping flat on your back may be due to uncomfortable mattresses or lumbar muscle problems, herniated lumbar discs, osteoporosis, osteoporosis and other diseases. 1. Uncomfortable mattress:If the mattress is too soft or too hard, it will cause discomfort in the lower back. Tension in the lumbar muscles leads to lumbar pain. 2. Lumbar muscle problems:The patient himself has damage to the lumbar muscles, with lumbar muscle strain, after lying down, the curvature of the lumbar vertebrae disappears, and the muscles are in a tense state, thus causing lumbar pain. 3. Lumbar disc herniation:The patient has lumbar disc herniation, and there will be the condition of lumbar pain when lying down. In addition to lumbar pain, such patients may also be accompanied by pain and numbness in the lower limbs. 4. Osteomalacia osteoporosis: middle-aged and elderly patients are likely to develop osteoporosis, osteoporosis and other diseases. Bone density and other related tests can be used to find the relevant causes. There are many causes of lower back pain, it is recommended to go to the hospital for X-ray, MRI and bone density test in time to clarify the cause of the disease for treatment. In the early stage, conservative treatment, physical therapy and medication can provide good relief.