People are measurably taller in the morning than at night.

The spine is cushioned by intervertebral discs, cartilage pads filled with a gel-like core that acts as a shock absorber between the vertebrae. Over the course of a day spent sitting, standing, and walking, gravity gradually compresses these discs, squeezing out small amounts of fluid.

That compression adds up to a measurable difference in height — commonly around one centimeter, or roughly a third to half an inch — lost between waking up and going to bed. Lying down overnight takes the pressure off the spine, letting the discs reabsorb fluid and rehydrate, which is why people are reliably a bit taller first thing in the morning. Astronauts experience an exaggerated version of the same effect in microgravity, where the total absence of spinal compression can leave them a few centimeters taller after weeks in space.