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It's just an observation. I don't know if there is actually a physics theory behind it. I am looking for an intuitive answer.

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    $\begingroup$ And why is it even longer when they hang by their feet (or their neck, for that matter)? ;-) $\endgroup$ Nov 6, 2021 at 22:25

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It’s because people are squashy.

Specifically in this case, it’s the squashy discs between the vertebrae which compress while a person is vertical, but can expand while a person is horizontal. The same expansion happens in zero gravity: astronauts return to Earth taller than they left. The nighttime difference is half-centimeter/quarter-inch for most people.

But at some level, all matter is compressible. The amount of vertical compression in Earth’s gravity is a question of the material’s strength and its rebound time constant.

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