Really, a nightmare just really has to evoke some sort of, we call it, dysphoric emotion or something uncomfortable. You could be sad, you could be unhappy; you could be scared, anxious. But traditionally, the definition is you have to awaken from this nightmare.
Emotional release by itself, no matter how "real," "honest," etc. the emotion may be, is never enough to create a character. . . such release has no artistic form.