“What causes pain in the back at night?”

Although there are several options re the cause of back pain, at night, most common may be mild spinal stenosis. While awake & active the pain is not as noticeable. Almost all musculoskeletal pain is worse at night when it seems your brain has less to keep it occupied & focuses on inflammatory things, pain associated areas. If not relieved by 7-10 days of an anti-inflammatory over-the-counter medication, Aleve or Advil (if these are safe for you to take), you may want to see an orthopedic surgeon for evaluation.

There are a lot of causes but nigh time pain worries us a bit. You need to see a specialist or your PCP. MRI might be needed.

Usually muscular.

Back pain at night (nocturnal back pain) can be something of a serious nature. It can be a tumor affecting the spine, an bone infection (osteomyelitis) or an inflammatory arthritis called Ankylosing Spondolytis (AS). AS is a condition where your spine can fuse together. After ruling out these conditions look to potentially have it be more "mechanical" in nature.