What occurs overnight to the intervertebral discs?

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During rest, the spine is unloaded, and the intervertebral discs rehydrate. The nucleus pulposus contains water-attracting proteoglycans, so when loading is reduced overnight, water diffuses back into the disc from surrounding tissues through the endplates. This increases the fluid volume inside the disc, which also causes the disc to regain height by morning. So the primary change is an increase in fluid volume. The drop in hydrostatic pressure due to unloading accompanies this process, and the overall effect is a slight height increase, but the best single description is the increase in fluid volume.