Lower back pain in the second half of the night, but during the day is fine, mainly consider the following reasons: First, the bed is not suitable: for example, sleeping in a soft bed, in a soft bed, the spine may be subsided, the lumbar physiological curvature will be changed, sleep often make the lumbar muscles are in the contraction, high tension state, may be manifested in the lumbar pain. Second, ankylosing spondylitis: ankylosing spondylitis is the manifestation of lower back pain, very obvious at night, sometimes even wake up with pain, and in the daytime just get up through the lumbar activities, stretching, etc., the pain can be relieved significantly. When the patient has related symptoms, it is still necessary to carry out X-ray of the lumbar region, X-ray of the sacroiliac joints, plus blood tests for HLA-B27 to make a clear diagnosis.