For this play I assume that you are a Dom having a willing sub/slave girl and you both want to experience or to experiment with sleepy sex. A good start would be to hypnotize her often before this scene so that she gets used to the state of a deep trance following your suggestions. Get her agreement for sleepy sex. You may adapt this probosal to your own needs. And then in one night as she sleeps and you get horny do the following: Whisper very softly in a way that it is hard to hear. Whisper so gently that she does not awake from it.