In the upright posture, gravity compresses us. Sitting, lifting and running also cause compression. When you lie down, gravity is no longer compressing you and all your fluid joints (biggest are the discs) will attempt to fill back up/expand. At the same time and also at night, your muscles stop working, begin to cool and shorten. This shortening will be more pronounced if your muscles are too tight to begin with, and will prevent the natural imbibing (taking in/drinking) of fluid. This will impair/minimize and fight the expansion causing stiffness and vulnerability of the back in the morning. Great things can happen in your spine and other joints while you sleep but only if you "leave the door open" and invite the fluid in. Doing this will minimize the risk of acute flare-ups/incidents that come "out of the blue" doing nothing more than bending to put your socks on or turn your head to look at the alarm clock.
