Your spine is comprised of 33 vertebral bones, neatly stacked from largest, at the bottom, to smallest, at the top. Fluid-filled intervertebral discs separate each vertebra and act as cushioning shock absorbers that support your upright posture. The squishy discs lose some of their fluid throughout the day. Thus, at the end of the day, your spine is slightly compressed from its natural state. In healthy individuals, the intervertebral fluid naturally replenishes overnight with proper rest and hydration.