Why does this occur. The answer is that the intervertebral discs, or spinal discs, are composed largely of water. The discs rest between the spinal bones, or vertabrae. These intervertebral disca are compressed as we stand and walk--it is our own body weight that compresses the intervertebral disc. When you sleep at night, the compressive pressure is relieved, and the discs expand again. While the difference is small, it is a fact!