Answer:
I'm tall in the morning and short in the noon. I disappear at night but I will be back soon.
What does everyone know how to open but not how to close?
What has two spines and a lot of ribs, and carries much but never moves?
What is the thing that stays the same size, but the more it's used the more it decreases?
I weaken all men for hours each day. I show you strange visions while you are away. I take you by night, by day take you back. None suffer to have me, but do from my lack.
Sometimes I am white, sometimes I am dark. Most people love me. What am I?