One reason is to do with the discs in your spine. While you are sleeping, compressive forces on your spine are reduced and your discs absorb water. This increases their size, making you on average 19mm taller by the morning. It also makes them slightly less flexible and therefore a little harder for you to put your socks on.As you move around your discs experience compressive forces, squeezing some of the water content out, making them slightly more elastic and flexible.By lunchtime your lower back is on average 5 degrees more flexible, but you will have lost the height you gained overnight.