Not sure if the phobia is just of dreaming you're falling, or dreaming of falling down stairs...either way it does convey the dream feeling - or nightmare in this case.
Technically, I would have liked to see more of the person. The softness is well executed to give the dreamy feeling. You're use of B/W for this as a good decision in my mind. As for composition, I think the crop could have been tighter along the bottom of your image. When I scroll, and keep the image showing only from the bottom step up, it looks better - more balanced.
I'm sure I've had this nightmare! Nice imagery here, you can sense what's going to happen next and you want to stop it. A nice use of blur, however I would have like to see more sharpness on the legs of the unfortunate one.
Sorry, I just don't get this. Doesn't convey nightmare, but perhaps a gentle "floating" dream. I guess the soft focus is to make this dream like, but looks boring rather than fearful.