You have probably heard this one before, but never been able to observe and confirm it yourself. This is probably because you are extremely groggy and inattentive in the morning, and you forget about it completely. Also, the increase in height is usually very small (around one centimetre). If you really want to see it for yourself though, there is a very easy method you could use. At night, just before sleeping, stand straight against a wall and ask someone to mark your height on the wall with a pencil. Then, as soon as you get up in the morning, stand in the exact same place and ask someone to mark your height again. If marked correctly, you should be able to see the difference in heights.
The spine is made up of several vertebrae that are separated by discs of cartilage. This cartilage allows the spine to move, rotate and extend. In the day, you spend most of your time sitting or standing. In these positions, the vertebrae and discs of cartilage are compressed into each other due to gravity. By the night, this compression causes the spine to reduce in length by a small amount. This amount varies from person to person, and it also depends on the lifestyle of the concerned person. At night, when you lie flat on a bed, the vertebrae and discs cartilage are given the chance to relax and retain their original positions. Over the night, they gradually decompress, and the full length of the spine is restored. Therefore in the morning you are slightly taller than you were at night. Of course, this will not work as effectively if you sleep in a sitting position, or if you sleep in a position where the spine does not have the chance to decompress.
So for all those people who feel they are too short, remember to get your height measured in the morning. That way, you could gain a few millimetres!