It means "are you still awake?" and this is the only meaning I know. It can also be used like this: What are you doing up so late? Are you up yet? Why are you up so early?

nozturk Let's see I can think of five. There probably are a few more. Are you up?.....Are you awake? Are you up?....Something a pilot might ask another pilot. Are you in the air? Are you up?....Are you ready emotionally for the challenge ahead? Are you up?....Are you ahead in the game? We are up 14 to 7. Are you up?...Is it your turn? Who's up to bowl on lane 4?

Are you up. 1. Have you got up yet? = Are you up, yet? 2. Haven't you gone to be yet? = Are you still up?

Basically it means "are you awake". However, you can use it in a different manner. Example: 1. What are you up to? 2. Are you up to much? 3. Are you up for...?