Sleep EEG in chronic insomnia disorder

Chronic insomnia disorder is among the most prevalent sleep disorders, and decades of research investigated about its diagnosis and treatment. Yet, the pathophysiological mechanisms underlying insomnia symptoms remain only partially understood. Contemporary conceptual frameworks characterize insomnia as a 24-hour disorder, marked by persistent physiological, cognitive, and emotional hyperarousal, resulting in distinct insomnia phenotypes. However, objective sleep assessment through traditional polysomnography provided limited insight into the mechanisms underlying insomnia disorder. In contrast, the application of advanced analytical techniques to both sleep and wake EEG recordings holds promise for this purpose, with quantitative EEG metrics and sleep microstructure features increasingly recognized as potential biomarkers of insomnia pathophysiology and symptom expression. On the centenary of the first EEG recordings, this narrative review aims to frame and summarize current evidence on EEG applications in insomnia research within the context of modern clinical models. After reviewing contributions from EEG studies conducted during both wakefulness and sleep in characterizing hyperarousal, sleep instability, and sleep misperception, recent findings on EEG-based markers of insomnia treatment response are presented. Finally, directions for future EEG research on insomnia are proposed, building on past achievements and advancing toward precision treatment and clinical translation.