ANSWER:
Day dreaming usually happens when a person thinking about something and he is not asleep. This means that he is bringing it up intentionally.
Generally speaking, one is not held accountable for the thoughts that go in his mind as long as he doesn’t talk about it or act upon it. However, this is one of the steps of satan as he gets a person ready to fall into sin bit by bit. You must try and avoid this and fight it.