Why do i have back pain only at night when i lay down flat?

Back pain: Usually while lying on the back, the lumbar spine arches a bit more, which creates more pressure on the small facet joints in the spine causing pain if they are inflamed or degenerated. Try a pillow under the knees which will reduce this arching and help relieve some pain. Long term procedures can help such as radiofrequency to numb up the joints. Pain physicians who are properly trained can help.