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Background Frequent nightmares show signs of hyperarousal in NREM sleep. Nevertheless, idiopathic nightmare disorder is considered a REM parasomnia, but the pathophysiology of REM sleep in relation to frequent nightmares is controversial. Cortical oscillatory activity in REM sleep is largely modulated by phasic and tonic REM periods and seems to be linked to different functions and dysfunctions of REM sleep. Here, we examined cortical activity and functional synchronization in frequent nightmare recallers and healthy controls, during phasic and tonic REM. Methods Frequent nightmare recallers (N = 22) and healthy controls (N = 22) matched for high dream recall spent two nights in the laboratory. Phasic and tonic REM periods from the second nights’ recordings were selected to examine differences in EEG spectral power and weighted phase lag index (WPLI) across groups and REM states. Results Phasic REM showed increased power and synchronization in delta and gamma frequency bands, whereas tonic REM featured increased power and synchronization in the alpha and beta bands. In the theta band, power was higher during tonic, and synchronization was higher during phasic REM sleep. No differences across nightmare and control participants or patterns representing interactions between the groups and REM microstates emerged. Conclusions Our findings do not support the idea that abnormal REM sleep power and synchronization play a role in the pathophysiology of frequent nightmares. Altered REM sleep in nightmare disorder could have been confounded with comorbid pathologies and increased dream recall, or might be linked to more specific state factors (nightmare episodes).

Introduction: About 2-5% of the general population is suffering from nightmare disorder. Nightmares are associated with several negative consequences but can be effectively treated with cognitive-behavioral therapies. Imagery rescripting (IR) and imaginal exposure (IE) are commonly identified as active treatment ingredients of psychological nightmare treatments. However, it remains unclear which specific therapeutic elements are responsible for the beneficial effects on nightmare symptoms. With this randomized controlled trial, we aimed to investigate the isolated therapeutic efficacy of IR and IE for nightmares.Materials and methods: In this trial we randomized 104 patients with a primary DSM-5 diagnosis of nightmare disorder into an IR, IE, or wait-list control group. In the intervention conditions all participants received three weekly 60 minutes individual treatment sessions. The treatment conditions only consisted of IR or IE and did not comprise of treatment elements such as extensive psycho-education, relaxation and safe-place exercises, or nightmare journals.Results: Post-test results showed that compared to WL, both interventions effectively reduced nightmare frequency and distress, as well as other associated symptoms such as the number of nights with nightmares, insomnia complaints, and dysfunctional nightmare beliefs. These results were sustained at three- and six-month follow-up.Conclusions: Given that the observed effects of IR and IE were comparable to those observed in meta-analyses for other psychological nightmare treatments, we conclude that IR and IE both seem to be active treatment components in nightmare therapies. Future research is needed to examine the potential superiority or equivalence of IR and IE in the treatment of nightmares. In a similar vein, future studies should investigate the potential beneficial effects of combining IR and IE.