In this article a differentiation is made between nightmares and reexperiences after psychotrauma. The psychotraumatic experience can influence the dream contents in such a way that REM-anxiety dreams or REM-nightmares develop. Besides, there is the symptom of the nocturnal reexperiencing in which a complete or partial replication of the psychotraumatic event takes place.

From sleep-physiological research it appears that the latter has similarities with parasomnia, originating in dissociation of circadian rythms as well as dissociation of activation of cerebral functions in different sleep phases. Examples are incubus, sleep panic disorder or REM-sleep behaviour disorder. Also, the report of the dream contents differs in these two phenomena.

The differentiation between posttraumatic nightmares and posttraumatic reexperiencing forms an explanation of the diverging sleep-physiological findings.

Last, some consequences for diagnosis, treatment and research are outlined.