we are a little bit taller in the morning than in the evening because during the day the cartilage in our knees and other areas slowly compress during normal activities, but when we go to sleep and rest the cartilage goes back to normal. On average we are about 1cm taller the morning than in the evening.
The other fact to this phenomenon is that the joint capsules lose some of their synovial fluid and the connective tissues around the joints tend to become compressed during the day from activities and try to counter the effects of gravity.
Also, the discs of the spine do the same thing. During the night, they absorb more fluid making them thicker and the person taller. While walking or being upright during the day, they slowly lose some of this fluid and become thinner.
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