It's such a mystery indeed to relate how a man can dream a nightmare and find a key and wash blood off his hands and not know what he has done, such a mystery that the story is told twice in the great film noir era because this same story was told in the film Fear In The Night (1943), also a great example of the style and its obsessions.
Essentially Nightmare (1956) is a tale of paranoia, murder and psychology, making of it noir mania in a deluxe package. Cracking weird action drives our hero played by Kevin McCarthy, doing a bit of what he was to later perfect in Invasion Of The Body Snatchers.