can the story be a nightmare? why?

All Poe's short stories can be considered nightmares. That bulging eye from The Tell Tale Heart still creeps me out. Usher is no different. We have a sister (Madeline) locked up alive in a tomb, sudden noises, return of the dead or undead...That's the stuff of nightmare. Was it a dream? That depends on your interpretation. Was poor Madeline dead or alive? That's the stuff you can figure out in your own dreams.... or nightmare!