Lying down reduces the pressure inside the spinal discs, letting them absorb body water and expand.  That’s the reason people are a little taller in the morning. If a large mass, such as a tumor, is within the spinal canal, this lengthening of the spine pushes across the mass causing increased back pain.  Some people with this symptom find relief by sitting and sometimes prefer to sleep sitting. This can be an ominous symptom.