Why do you go to bed every night?

Q: Why do you go to bed every night?

A: Because the bed won't come to you!

This is a joke in the standard riddle format. The humor in this joke is the false assumption that the listener makes when the jokester is setting up the joke - the false assumption is that the jokester is going to answer the question "Why is it important to sleep every night?" instead of "why do you move toward a bed every night?"

When the jokester answers the second question, the temporary confusion that the listener has is humorous.