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

Introduction: Insomnia disorder (ID) is characterized by signs ofcortical hyperarousal. Such high arousal level may be associatedwith the phenomenon called sleep state misperception (SSM): atendency 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 regulationprocesses. However, its role in ID and its possible relationshipwith SSM are still poorly investigated. The aim of the presentstudy was to assess quantitative and morphological KC alterationsin ID compared to healthy controls (HC), and their relationshipwith 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 completeda sleep diary following the final awakening. An index of totalsleep time misperception (TSTm) was calculated comparing objective(PSG) and subjective (sleep diary) measures of TST. KCs were manuallydetected from N2 sleep in 3 midline derivations (Fz, Cz, Pz). KCdensity, amplitude, and area under the curve (AUC) were computed.Macrostructural PSG variables, KC measures and TSTm were comparedbetween ID and HC. Finally, the relationship between TSTmand KC measures were assessed.Results: The analysis of sleep macrostructure highlighted signs of disruptedsleep in ID compared to HC. Moreover, the assessment ofTSTm pointed to a greater TST underestimation in ID compared toHC. ID patients showed lower fronto-central KC amplitude and frontalKC AUC compared with HC. Finally, lower centro-parietal KCamplitude was associated with higher TST underestimation.Conclusion: The present findings highlight the presence of morphologicalalterations of the KC, specific for the fronto-central region,where they are maximally expressed, and in absence of changes in KCdensity. Moreover, the correlational analysis suggests that thereduced KC amplitude, theoretically expression of impaired efficiencyof the sleep protection mechanism, is related to SSM.