There are lots of ways to add to your height. Girls, and some guys, will fluff their hair. Think Marge Simpson, or the beehive hairdos of the 1960s. Then there are high heels, or for men, the more discrete “lifts” for their shoes that supposedly disguise the fact they’ve suddenly grown two inches. Or you can just go to bed.
When you’re off your feet and lying prone, the spine is not under the compression caused by your weight and gravity. Because there is soft tissue (discs) between the vertebrae, the tissue is compressed by gravity, and your upright height is what most people measure. However, if you were to carefully measure your height while still in bed, or as soon as you get up, you’d find that it has changed by as much as 1/2 inch. But don’t go lengthening your pant legs, because the spine will compress again when you finally climb out of bed.
This oddity is one reason people who treat back disorders, will recommend firm mattresses. They’re needed to support the back as it relaxes and decompresses at night. It’s also the reason why astronauts can grow as much as three inches during a space mission.
In space, there is no gravity, so as soon as a capsule lifts off, the compression on an astronaut’s spine ceases, and the softer tissue of the discs will expand with the extension of the vertebrae.
The phenomenon was first observed during the Skylab missions of more than 20 years ago. Since then, adaptations in both uniforms and space capsules have been made to allow for the “growth” of the occupants.