Nightmare disorder shows reduced slow oscillation-dominant spindle coupling in NREM sleep.

Nightmare disorder is historically conceptualised as a rapid eye movement (REM) sleep parasomnia, but evidence also points to altered non-rapid eye movement (NREM) sleep physiology. Here, in a retrospective case-control study, we analysed overnight polysomnography from 26 adults with nightmare disorder and 32 controls using a harmonised event-based EEG pipeline. Nightmare disorder showed reduced frontal slow-oscillation-spindle coupling, whereas delta-spindle coupling was preserved, yielding lower coupling dominance than in controls (0.137 ± 0.083 versus 0.238 ± 0.096; p = 6.8 × 10-5). Stage-adjusted K-complex density across the first 6 h from sleep onset was also lower in nightmare disorder (0.463 ± 0.345 versus 0.723 ± 0.475 events per minute of available N2/N3 sleep; p = 0.019), whereas peak-timing differences were not robust. These findings suggest altered NREM sleep microarchitecture in nightmare disorder rather than a phenotype confined to REM sleep, and warrant prospective confirmation in harmonised, clinically phenotyped cohorts.