Why are we taller in the morning than in the night?

The spine compresses during the day with the weight pushed down on it. During the night it expands again, as lying down takes the weight back off. Depending on your height, you gain/lose a few cm during this process. If you ski, it's quite an important thing to remember as the risk of injury to the spine increases the longer you are out for (as the spine is compressed and less flexible).