Because the cartilage in our knees slowly compress during the day.
- But when you go to sleep and rest the cartilage goes back to normal. On average we are about 1 cm taller the morning than in the evening.
- The other component to this phenomenon is that the joint capsules loose some of their synovial fluid and the connective tissues around the joints tend to become compressed throughout the day from activity and trying to counter the affects of gravity.
- Also, the discs of the spine do the same thing. During the night, they resorb more fluid making them thicker and the person taller. While walking or being upright during the day, they slowly lose some of this fluid and become thinner.
- The spine is made up of 33 overlapping vertebrae separated by intervertebral discs made of cartilage. These discs allow for movement, flexibility, extension, and rotation in the spine. When you sleep, your spine is no longer subjected to the pressures of gravity, and your body is able to replenish lost fluids between your intervertebral discs, which stretch and relax. The excess fluid provides up to a half inch more of height upon waking.
- This same force is at work when astronauts travel to outer space. According to Nasa research, astronauts gain up to two inches in height once they are freed from the gravitational pressure of Earth. Unfortunately, we don’t live on the moon, and the force of the earth’s gravity compresses our spines throughout the day, draining that extra fluid and shrinking height that extra half-inch.