Are you really taller in the morning than at night?

Your spine is a column of about two dozen small bones, and between them sit soft discs called intervertebral discs. These discs hold a lot of water. While you stand or sit, body weight presses on them and water slowly seeps out, so they get thinner. Each disc shrinks only a tiny bit, but with more than twenty stacked up, the total becomes noticeable. Lying down at night takes the pressure off, the discs soak water back in, and they regain their thickness. That is why you are tallest just after you get up.

In the 1980s, researchers in Liverpool, England, measured the height of eight young adults to the millimeter throughout the day. The gap between morning and night averaged 19.3 mm, about 1.1% of their height. The surprising part was the speed. A little over half of the day's loss happened in the first hour after getting up, and about 80% was gone within roughly three hours. Carrying heavy loads made it shrink faster, and resting while reclined brought height back faster than resting on your feet. At night, about 70% of the regained height came back in the first half of sleep. With so few participants, individual differences may be larger.

A common myth is taking "a good night's sleep makes you taller" as real growth. The extra length you have in the morning goes away by evening; it is a change that comes and goes day by day. So if your evening height reads lower than your morning height, there's nothing to worry about. In space the gap gets bigger. NASA says space station crew members can grow up to 3% taller and return to their usual height back on Earth. Without gravity pressing down, the spine straightens and lengthens. In daily life, if you want to track height changes, measure at the same time of day. When marking a child's height on a door frame, keeping the time consistent makes the record less jumpy.