By the time a 6-foot-tall person goes to bed at night, for example, she has lost almost 3/4 of an inch in height. Osteologists explain that gravity pulls on the body throughout the day, compressing the discs between the vertebrae in the spine. To a lesser extent, the same compression happens in the leg joints. Though the changes are small, they add up. After a long day of walking around, the cumulative effect can be noticeable. A good night's sleep allows the body to recover and gain back that height lost during the day.
Age, however, is a more permanent factor in making people shrink over time. After age 40, the average person loses almost an inch of height every 20 years. A person who lives to be 80 would therefore lose nearly 2 inches of height. Such long-term loss is the result of the cumulative effects of gravity and the stress of being bipedal, both of which work together to shrink the skeleton. At the same time, the discs between the vertebrae tend to dry out and lose mass over time, which also reduces vertical height. Physicians suggest that exercise and a diet rich in vitamins and minerals like calcium can reduce, but not eliminate, the tendency to lose height over the long term.Learn more about Human Anatomy