You can talk about sleeping if you want to. But my point was that you can also just say She had a dream in which her boss appeared to her and gave her an assignment. When you talk about a dream, we assume that the person is asleep, so you don't have to say so (unless you want to).
She reached her home and fell asleep immediately after changing her clothes. While asleep she had a dream in which her boss appeared to her and gave her an assignment.