Unaltered EEG Spectral Power and Functional Connectivity in REM Microstates in Frequent Nightmare Recallers: Are Nightmares Really a REM Parasomnia?

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' re-cordings 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). (c) 2020 Elsevier B.V. All rights reserved.