The inner core of your discs is made of jelly-like material that provides the flexibility and "cushioning" in your back. When you are sitting, standing, or exercising, which I'll term "weight-bearing activities", fluid is squeezed out of your discs and into adjacent soft tissue (just as you would squeeze moisture out of a sponge). As a result, your discs lose some of their moisture and height. To prove this fact, measure yourself in the morning and then again at night. You will lose 1/2" to 3/4" in height by the end of the day. This lost fluid translates into your discs loosing some of their cushioning effect.
When you are sleeping, "a non-weight bearing activity", your discs (or "intravertebral sponges") expand as they soak up fluid and nutrients and increase the length of your spine by as much as 3/4". But you don't gain the full height back, accumulating to a total of 1/2" to 2" in height throughout your lifetime.
When your discs are compressed and thinned, your vertebrae potentially place more pressure on these nerve roots. More pressure equals more pain. As you relax, your spine begins to stretch. The space between each vertebra will increase, thus decreasing the pressure on the discs between each vertebra. This encourages fluid movement back into the disc, helping to keep your discs plump and happy and decreasing the pain in your back.
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In fact, the only time in your life when you are giving your discs a break is when you are inverting. See graph. The medical study that generated this graph measured the pressure inside the 3rd lumbar disc-it was assigned a baseline pressure while standing of 100%. The study reported that even when you are lying down, the disc pressure remained at 25%. The hundreds of ligaments and muscles that encase and mobilize the spine act like a bunch of rubber bands holding the spine in compression equal to 25% of standing body weight. Inverting to 60° is necessary to reduce the disc pressure to zero. Inverting is the most effective posture that allows your discs to recover from the constant pressure placed on them during your daily activities.