It’s grammatically correct and naturally phrased, but there are some possible issues. First of all, the word “must” usually (90% of the time) is used for speculation, so the meaning is most likely that the speaker is fairly sure that the person addressed is sleeping. Actually, the continuous aspect “be sleeping” (which has the same meaning as “be asleep”) makes any other interpretation very unlikely. If we meant “must” as in “have to”, we would say “you have to sleep” (in theory, we could use “must” with this meaning, but it would be unusual, and ambiguous). Finally, if we interpret the sentence in the most usual way (as expressing the speaker's belief that the person addressed is sleeping), it’s a bit funny, since we don’t normally speak to people we know to be asleep.... Of course, it could be a polite way of asking the other person if they are asleep or not.