Okay I just found this and it sounds interesting as hell.

Here's an idea: It's believed that regaining control after you become a nightmare is next to impossible, and effectively unheard of. Suppose someone did come back as that through some means? In the depths of their dream they found a foothold and slowly rebuilt themselves up, taking back control of their body but becoming physically fucked up in the process. You can just [i]tell[/i] they're a dreamer.

Psychologically they're like a broken mirror made whole again. It's one object and the cracks are filled, not removed.