Q: Why did the boy run around his bed?
A: Because he was trying to catch up on his sleep!
Explanation: To catch up on something (such as sleep) is to do something you have not had time to do. You catch up on sleep by sleeping more after a time of little sleep. You can also catch up on the latest news by reading/ watching today’s news (or the newest tweets on Twitter, for example). Someone might say, “When I returned from vacation I had to catch up on the news/ my emails/ my homework …
To catch someone is to chase after that person until you can touch him or her. You might catch someone in a race; a police officer might catch a criminal. (You can also catch a ball but that is a different meaning).
This joke is funny because it plays with the different meanings of the word catch and the phrasal verb catch up on.
Speaking of catching, do you know the story of The Gingerbread Man?