"Should you not be sleeping now?"

Unlike English, Danish doesn't have separate forms for Present Simple and Present Continuous: du sover can be translated both as "you sleep" and "you are sleeping". So this sentence also translates to "Should you not sleep now?"

A hint - I use Present Simple in "type in English" tasks all the time to save me the typing of modal verb ("are"/"am" and this case "be") and the "-ing".

Skulle is the infinitive, past tense, or (important here) the conditional form. It most directly translates to "should".

Skal is present tense and is more around "shall" or "have to".