Asleep is an adjective in Are you asleep? while sleeping is a verb in Are you sleeping? Use asleep if you want to emphasize the state and sleeping if you want to emphasize the action.

Minimal difference. In your example, "sleeping" is an active participle, so it's being used as a verb to form the past continuous. "Asleep" is simply an adjective, indicating a state. In practice, the meaning is nearly the same.

As others have said it is a verb form. I was sleeping on the couch (doing now) and I fell asleep on the couch (did ) are the same just different slightly in meaning.