Because hallucinations and visions are experienced with so much emotion they seem more real than life and it becomes difficult to discern what is reality and what is not. The movie skates along that overlap of reality and vision.
Can nightmares drive a person to insanity, to the point he sees visions?
When nightmares become real, what is the difference between nightmares, visions, and reality?
Music by Max Richter: The Nature of Daylight