Lumbar pain when lying down may be related to the following reasons: 1. It may be caused by muscle spasm in the lumbar region. When the patient takes the prone position, the muscles of the lumbar region are relaxed in the early stage, but after a long time there will be muscle spasms on both sides or on one side of the lumbar region, which will cause pain here. 2. It may be caused by the slippage of the lumbar vertebrae. Once this disease strikes, instability of the lumbar spine will occur. The patient takes a prone position, the front vertebrae may slip forward, which in turn produces nerve root compression and irritation inside the lumbar spine, resulting in pain. 3, may be caused by a herniated lumbar disc. The patient’s nerve roots are often compressed and stimulated when the disease strikes, and at this point the patient takes a prone position, which may cause the nerve roots to be compressed more severely, leading to pain. 4, may also be caused by old injuries to the lumbar spine, such as acute lumbar sprains, or fractures of the spinous process or transverse process, which may also occur when the patient takes a prone position.