Reactivating a Relaxation Exercise During Sleep to Influence Cortical Hyperarousal in Individuals With Frequent Nightmares.

High-frequency EEG activity during sleep, reflecting cortical hyperarousal, has recently been incorporated into aetiology models of nightmares. However, its precise role remains insufficiently understood because experimental approaches to manipulate hyperarousal during sleep are limited. In the present study, we investigated whether re-presenting a relaxation-associated odour cue during sleep to reactivate a previously practiced deep-breathing relaxation exercise reduces cortical hyperarousal as indexed by beta (16.25-31 Hz) and gamma (31.25-45 Hz) activity and, consequently, nightmare symptoms. Twenty-five participants (21 female, mean age (SD) = 23.12 (3.70) years) experiencing at least one nightmare per week completed a 1-week deep breathing relaxation training paired with an odour cue. In a randomised, blinded, crossover design, either the relaxation-associated odour or a control odour was presented across two laboratory nights. Odour-based reactivation did not influence beta or gamma power but was unexpectedly associated with a significantly reduced spindle count and density. Spindle count during reactivation nights correlated positively with subjective wake-after-sleep-onset. Nightmare symptoms were unaffected. These findings suggest that odour-cued reactivation of a relaxation exercise did not modulate cortical hyperarousal or nightmare symptoms, despite altering spindle activity. The functional significance of this spindle effect remains to be determined. Future studies should employ longer reactivation periods and alternative interventions targeting cortical hyperarousal to further clarify its causal role in nightmare aetiology and the therapeutic potential of sleep-based interventions.