An interesting natural phenomenon occurs each night we go to sleep—we all get a little bit taller—and as the day goes on, we all get a little shorter. Even though what happens is complex, the explanation is rather simple.

You probably don’t feel taller in the morning after waking from a good night’s sleep, but you’ve actually grown. The reason lies in the vertebral column, specifically in what is known as the nucleus pulposus, or the inner part of the vertebral disc. The discs in the spine are composed of this gelatin-like material which provides cushioning and protection to the spine. It’s your body’s shock absorber, and with the pounding your vertebrae take during the day with walking, running, bending, lifting, and sitting, it needs time to rest and rejuvenate.

During the night when there is no load placed on your spine, fluid is slowly diffusing into the discs in a passive process called imbibition. Without forces compressing the spine, which includes gravity when you’re standing or sitting, discs grow in size due to osmotic pressures. An analogy to consider is that of a balloon with extremely tiny holes in it that is filled with gelatin and water sitting in a tub of water. When the balloon is compressed, water seeps out, decreasing the volume in the balloon. When the compression is released the reverse happens. Each disc goes through this process at rest and increases in height by a small amount.

The vertebral column is made up of 24 vertebrae, and between each vertebra is a disc, 23 in all. The height of the spine increases when you add up the amount that each disc increases in size at night. This makes you about one and a half to two centimeters taller in the morning. As the day progresses, the discs slowly lose some of their height due to compressive forces, and you’re back to being shorter at night again. The discs can be reduced by up to 15% from compressive forces that happen during the day.

While we get shorter and taller day by day, over the span of our lives we do shrink. You weren’t just imagining things when grandma appeared shorter and shorter as she aged. The reason for this is the water content of the discs typically decreases with age. This results in a much narrower disc that doesn’t have the ability to “refill” fully. As before, when you add this up across the entire vertebral column, the loss of height becomes significant. Add to this the natural changes in the curves of the spine as we age, and the change in height becomes even more significant.

There are other types of disc problems that can also affect changes in height or give an idea of the health of the discs. One of the common questions asked to someone with back problems is if they have pain or stiffness in the morning after waking. Since the disc is at its fullest point during this part of the day, this can help test the indication of a bulging or herniated disc. Often the pain subsides as the day goes on since the pressure in the disc is decreased and the protrusion of the disc on the nerves in the spine are also reduced.

Even if there isn’t a problem present, many people are just stiff in the morning. Again this is because the disc is at its fullest pressure, and as you begin to move and compress those discs, the stiffness decreases. Of course there can be other things in play and in the mix such as ligament, muscle, or soft tissue tightness of the back and neck. A degenerated disc or a thinning disc can also cause problems in the spine and with how well the disc fills at night.

So why do the discs go through this every night you may ask? The diffusion of fluid provides nutrients to the nucleus pulposus, the central part of the disc, and the fibrous rings that surround it to keep the material in place, called the annulus fibrosus. A lack of motion and physical activity also decreases disc nutrition, but too much overload on the disc leads to those other disc problems mentioned before.

So there is a fine line to walk with disc health. Modern stressors contribute greatly to problems of the discs in the back. Excessive sitting, low amounts of physical activity, and poor posture all reduce the health of the discs. Loading of the spine and movement allows unloading later on and the ability for the discs to revitalize themselves. So keep moving to make sure you keep growing by the time the sun comes up in the morning.

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