Distinct hypervigilance profiles in sleep-onset insomnia with and without psychiatric comorbidity

Objectives: Sleep-onset insomnia (SOI) is characterized by difficulty initiating sleep and is frequently comorbid with psychiatric disorders. Despite its prevalence and clinical impact, pathophysiological biomarkers and a clear nosological framework remain lacking. Conventional polysomnographic (PSG) measures offer limited insight into the continuous dynamics of arousal across the night. We used high-resolution EEG markers to characterize and compare hypervigilance in isolated insomnia versus insomnia comorbid with affective symptoms. Methods: We retrospectively analyzed PSG recordings from 2,952 individuals. Alongside theta/alpha ratio dynamics and micro-sleep detection, we developed an intrinsic Vigilance Score (iVS), a continuous probability-of-wakefulness index calibrated individually on each participant's own sleep-wake distribution. Individuals with and without SOI were compared, and SOI subgroups with and without depressive or anxiety symptoms were further examined. Results: Strikingly, hypervigilance was more pronounced in isolated SOI than in SOI comorbid with psychiatric symptoms, particularly depressive symptoms, pointing to partially dissociable mechanisms across subtypes. More broadly, SOI was marked by persistently elevated EEG-defined vigilance extending from wakefulness through the sleep-onset period and across all sleep stages, including N2, N3, and REM sleep, accompanied by arousal instability at sleep onset and delayed accumulation of deep sleep. These alterations remained largely undetected by conventional PSG macrostructure. Conclusions: Continuous, individually calibrated EEG markers capture microstructural alterations invisible to standard staging and reveal greater hypervigilance in isolated than in comorbid insomnia. These findings support a conceptualization of SOI as a disorder of persistent vigilance dysregulation.