Spinal height does change throughout the day. This is mainly due to disc height. The discs in between the vertebrae are fluid-filled and act as a cushion against compressive forces on the spine.

After rest and especially after sleeping for six to eight hours, the discs are fully restored. For this reason people are at their tallest after getting up from bed. Activity and the upright posture slowly push some of the fluid out of the discs. When this happens height may decrease slightly.

This spinal shrinkage may be counteracted by muscle activity of the paraspinal muscles along each side of the spine. The normal adult is not usually aware that any of this is happening. In the person with a bulging or herniated disc, the fully plumped up disc can put increased pressure on the nearby spinal nerves. The result is greater back pain in the morning.