If you've ever been measured at the doctor's office and thought, "That can't be right," you might have a point. The number on the chart depends partly on the time of day. Most of us walk around a little shorter in the evening than we were when we rolled out of bed.
The short answer: you lose a small but measurable amount of height over the course of a day, often around half an inch. In one study of 100 children, height dropped by an average of 1.54 centimeters, or about 0.6 inches, between getting up in the morning and late afternoon. Lying down for a good stretch, like a night's sleep, brings it back.
The Cushions in Your Spine
Your backbone isn't one solid rod. It's a stack of vertebrae, and between most of them sits a cushion called an intervertebral disc. There are 23 of these discs in the human spine: 6 in the neck, 12 in the middle back, and 5 in the lower back.
Each disc has a tough outer ring and a soft, gel-like center that acts as a shock absorber. That gel spreads pressure in every direction when the spine is under load. It also holds on to water, which helps keep the disc plump.
Gravity Does the Squeezing
When you're upright all day, standing, walking, and carrying things, your body weight presses down on that stack of cushions. Over hours, the discs are thought to give up a little of their fluid and flatten slightly. Add up a tiny change in each of 23 discs and you get a difference you can measure.
Activity matters too. Height measured right after exercise tends to come out lower, while height after lying down for a significant time tends to come out higher. That's why the most consistent readings come from measuring at about the same time of day.
What Happens Without Gravity
Astronauts show how big a role gravity plays. In weightlessness, the human skeleton fully stretches out within about a month, and height increases by roughly an inch. A night in bed works on the same idea in a much smaller way. With your weight off your spine for several hours, the discs get a chance to recover before the next day begins.
That extra height isn't all good news. Back pain is a common complaint among space travelers as their bodies readjust. And the moment they return to Earth's gravity, the stretching starts to reverse.
The Slow Shrink Over a Lifetime
There's also a longer, slower kind of shrinking. With age, the gel inside the discs gradually dries out and loses some of its shock-absorbing ability. This general shrinking of the discs is part of why many people get shorter as they grow older.
Depending on genes, sex, and environment, this loss of height can begin in middle age for some people, and it's nearly universal in the very old. Changes in posture and in soft tissues add to it. If you notice you've lost a noticeable amount of height, it's worth mentioning to your doctor at your next visit.
So if you want to win a height contest, schedule it for breakfast. You'll stand at your tallest first thing in the morning, then lose a little as the day goes on. The good news is that a good night's sleep gives most of it right back, ready to do it all over again tomorrow.
