We are taller in the morning than in the evening. This is because during normal activities during the day the cartilage in our knees and other areas slowly compress, but when you 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 component to this phenomenon is that the joint capsules loose some of their synovial fluid and the connective tissues around the joints tend to become compressed throughout the day, this is from activity and trying to counter the affects of gravity.
Also, the discs of the spine do the same thing. During the night, they reabsorb 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.