The K-complex in insomnia disorder: Morphological alterations and relationship with sleep state misperception

Introduction: Insomnia disorder (ID) is characterized by signs of cortical hyperarousal. Such high arousal level may be associated with the phenomenon called sleep state misperception (SSM): a tendency of ID patients to underestimate total sleep time (TST). The K-complex (KC) is a sleep EEG feature commonly considered a “guardian” of sleep, involved in sleep protection and arousal regulation processes. However, its role in ID and its possible relationship with SSM are still poorly investigated. The aim of the present study was to assess quantitative and morphological KC alterations in ID compared to healthy controls (HC), and their relationship with SSM. Method: 19 ID patients (7 M; 42.42 ± 12.13 years) and 19 HC (9 M; 41.05 ± 11.86 years) performed a single night of polysomnographic (PSG) home recording (19 EEG derivations, EOG, and EMG) and completed a sleep diary following the final awakening. An index of total sleep time misperception (TSTm) was calculated comparing objective (PSG) and subjective (sleep diary) measures of TST. KCs were manually detected from N2 sleep in 3 midline derivations (Fz, Cz, Pz). KC density, amplitude, and area under the curve (AUC) were computed. Macrostructural PSG variables, KC measures and TSTm were compared between ID and HC. Finally, the relationship between TSTm and KC measures were assessed. Results: The analysis of sleep macrostructure highlighted signs of disrupted sleep in ID compared to HC. Moreover, the assessment of TSTm pointed to a greater TST underestimation in ID compared to HC. ID patients showed lower fronto-central KC amplitude and frontal KC AUC compared with HC. Finally, lower centro-parietal KC amplitude was associated with higher TST underestimation. Conclusion: The present findings highlight the presence of morphological alterations of the KC, specific for the fronto-central region, where they are maximally expressed, and in absence of changes in KC density. Moreover, the correlational analysis suggests that the reduced KC amplitude, theoretically expression of impaired efficiency of the sleep protection mechanism, is related to SSM.