Hypervigilance profiles in sleep-onset insomnia and psychiatric comorbidity

Study 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. Statement of Significance Insomnia is among the most common health complaints, yet clinicians still lack reliable biological markers to characterize it and distinguish its subtypes. This study shows that difficulty falling asleep reflects not merely a delayed onset of sleep, but a sustained state of heightened brain alertness that persists throughout the night and across all stages of sleep. Notably, this hypervigilance is stronger when insomnia occurs alone than when it accompanies depression or anxiety, suggesting that these conditions arise through partly different mechanisms. Because conventional sleep recordings miss these subtle dynamics, finer continuous measures of vigilance could improve diagnosis and guide treatments. Future work should test whether such markers predict treatment response and reflect everyday sleep experience.