Sleep stage transitions driven by K-complex: Electrophysiological patterns in insomnia disorder

Introduction: An electrophysiological explanation for mechanisms underlying Insomnia Disorder (ID) should be sought in sleep protection mechanisms that ensure sleep stability and continuity. The K-Complex (KC), a typical waveform of Non-Rapid Eye Movement (NREM) sleep stage 2 (N2), is a cortical event that plays a key role in sleep protection and as a forerunner of slow waves. This exploratory study aims to evaluate the transition probabilities between N2 epochs characterized by low (lKC) and high (hKC) KC frequency into deepening or lightning sleep stages, and to test differences between ID patients and Healthy Controls (HCs). Method: Twenty IDs (mean age: 43.5 ± 12.7; 7M/13F) and eighteen matched HCs (mean age: 41.6 ± 11.9; 8M/10F) were recruited and underwent a nocturnal polysomnography recording. The Markovian Chains methodology was applied during the First NREM sleep Cycle (FC) and the Total Sleep Time (TST). The transition probabilities were calculated between sleep stages according to N2 epochs with lKC and hKC frequency based on the median of both groups. Results: Wilcoxon signed-rank tests in the whole sample showed that during the FC, the probability of transitioning from hKC N2 epochs to NREM sleep Stage 3 (N3) was higher than the probabilities of transit from hKC to NREM sleep stage 1 (N1) (hKCàN3 vs. hKCàN1: 0.117 ± 0.104 vs. 0.019 ± 0.026; W = 361, p < 0.001) or to wake (hKCàN3 vs. hKCàwake: 0.117 ± 0.104 vs. 0.012 ± 0.021; W = 344, p < 0.001). Moreover, the same finding resulted considering the TST (hKCàN3 vs. hKCàN1: 0.059 ± 0.042 vs. 0.028 ± 0.018; W = 362.5, p < 0.001; hKCàN3 vs. hKCàwake: 0.059 ± 0.042 vs. 0.025 ± 0.017; W = 439, p < 0.001). We repeated the tests separating HCs and IDs, showing the same pattern of results. However, when comparing the probability of transit from hKC to N3 between the two groups, this was higher in HCs than IDs during the FC (ID vs. HC: 0.155 ± 0.104 vs. 0.081 ± 0.094; U = 242.5, p = 0.029), not attributable to group differences in the hKC percentage (U = 184, p = 0.704). Conclusion: These findings confirm the role of KC as forerunner of deep sleep, both in FC and in TST, regardless of the presence of ID. However, the transition probabilities from hKC epochs to N3 during FC were significantly reduced in ID patients compared to HCs, suggesting a possible breakdown of neurophysiological mechanisms related to KC in this condition.