Trauma & the Impact on the Nervous System
Eye Movement Desensitization & Reprocessing (EMDR) is a treatment modality created by Francine Shapiro (1987) for reprocessing our unresolved emotional pain after a traumatic or distressing event, such as a car accident. 

The theory of how distressing events are not fully processed in our minds and body after some distresing events occur is called Adaptive Information Processing Theory (AIP). For several reasons that may be unconscious our out of our awareness, (the traumatic memory or event) the brain has difficulty making sense of all the stimuli (the original picture, sounds, thoughts, feelings, and body sensations).

EMDR treatment, seems to work by engaging the parasympathetic nervous system which seems to have a calming effect on the nervous system's natural flight, fight or freeze response that is often triggered or heightened afer a traumatic event. Several methods and techniques can be taught to engage the parasympathetic calming response with EMDR, with the (the back and forth eye movements, listening to back and forth tones through a headset, or some form of tactile connection with the body) helps reprocess the memory and the associated experiences. ) seeming to be the most effective for helping to calm the stimulate the information and allow the brain to reprocess the experience.
 
That may be what is happening in REM or dream sleep. It is your own brain that will be doing the healing and you are the one in full control and this is done in a safe environment in session only.