You are about a centimetre taller in the morning than in the evening.
During the day, gravity squeezes water out of the discs between your vertebrae, and the spine compresses slightly. Overnight, lying down, the discs soak up fluid again.
Why is this true?
Astronauts show the extreme version: without gravity the spine stretches and they return from space several centimetres taller, at least until they have been back on Earth for a while.
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More about human body
Your own body is a reliable source of surprises. It replaces cells by the million every second, keeps bacteria that outnumber human cells, and runs sensory tricks that make the world feel smoother than the raw signal ever is. The facts collected here stick to mainstream physiology, avoid the wellness myths that circulate alongside them, and explain the underlying mechanism rather than just quoting a startling number.