The main causes of pain on the left side of the waist in the second half of the night include diseases of the waist itself and diseases of the urinary and gynecological systems. 1. Diseases of the lumbar region itself: mainly including lumbar muscle strain, lumbar disc herniation, lumbar spondylolisthesis, lumbar spinal stenosis, etc. Diseases of the lumbar region itself may cause edema and rigidity of the lumbar muscles, congestion and tension of the fascia, impaired local blood circulation, accumulation of acidic metabolites, which may indirectly stimulate the nerve roots or directly stimulate the sensory nerves that innervate the left lumbar region, resulting in pain symptoms. 2. Urological, gynecological system diseases: the left kidney is located in the left side of the lower edge of the 12th rib and the lateral edge of the erector spinae muscle in the kidney angle, kidney disease can also cause pain in the left side of the waist, such as left ureteral stones, kidney stones, left glomerulonephritis, and so on. In addition, if it is a female patient, certain gynecological system diseases can also cause radiating pain in the left side of the waist, such as pelvic inflammatory disease, left salpingitis, endometritis and so on. There may be other reasons for the pain in the left side of the lower back in the second half of the night, it is recommended to go to the hospital in time to improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.