Decreased Delta/Beta ratio index as a state-independent signature of sleep state misperception in insomnia disorder

Introduction: Sleep State Misperception (SSM) is the tendency of Insomnia Disorder (ID) patients to overestimate Sleep Latency (SL) and underestimate Total Sleep Time (TST). Literature exploring topographical components in ID with SSM is scarce and does not allow us to fully understand the potential mechanisms underlying SSM. This study aimed to evaluate the existence of sleep EEG topography alterations in ID patients associated with SSM compared to Healthy Controls (HC), focusing on the Sleep Onset (SO) and the whole night. Method: Twenty ID patients (mean age: 43.5 ± 12.7; 7M/13F) and 18 HCs (mean age: 41.6 ± 11.9; 8M/10F) underwent a night of polysomnography and completed sleep diaries the following morning upon awakening. Two SSM indices, referring to the misperception of SL (SLm) and the TST (TSTm), were calculated by comparing objective and subjective sleep indices extracted by polysomnography and sleep diary. According to these indices, the entire sample was split into 4 sub-groups: ID + SLm and HC-SLm; ID + TSTm and HC-TSTm. Results: Considering the SO, the two-way mixed-design ANOVA showed a significant main effect of Groups pointing to an increased beta activity involving anterior and temporoparietal cortical derivations (p ≤ 0.023) and to decreased delta/beta ratio in the whole scalp topography (p ≤ 0.040). Moreover, we found a significant (p ≤ 0.024) interaction effect for the sigma and beta bands. Post Hoc tests showed higher sigma and beta power in anterior and temporo-parietal sites during the SO period in IDs + SLm compared to HC–SLm. Considering the whole night, the unpaired t-test revealed in IDs + TSTm significantly lower delta power during NREM (p ≤ 0.035) and lower delta/beta ratio index during NREM and REM sleep (p ≤ 0.034) compared to HCs–TSTm. Finally, we found diffuse significant negative correlations between SSM indices and the delta/beta ratio during the SO (r ≥  0.589; p ≤ 0.011), NREM (r ≥  0.424; p ≤ 0.042), and REMsleep (r ≥  0.372; p ≤ 0.042). Conclusion: The main finding of the present study suggests that higher SL overestimation and TST underestimation are both related to diffuse cortical hyperarousal, interpreted as a state-independent electrophysiological correlate of the SSM, both during the SO and the whole night.