• EEG
• insomnia disorder
• Lempel-Ziv complexity
• NREM
• REM
• sleep

Background: Insomnia disorder (ID) has been associated with alterations in both slow-wave activity and REM sleep EEG patterns. In particular, the REM sleep instability hypothesis has been proposed as a novel framework for understanding ID. However, EEG studies on the topic are still limited. The diversity of spatiotemporal brain activity is reflected in EEG complexity, which may provide new insights into the informational content and dynamic structure of sleep in ID. This exploratory work aimed to explore slow-wave sleep and REM sleep dynamics in insomnia patients using EEG-based complexity metrics.
Methods: We analyzed overnight polysomnographic recordings from 45 insomnia patients and 50 age- and sex-matched good sleepers. Standard sleep architecture parameters were extracted, and EEG complexity was assessed using two Lempel-Ziv compression methods: (i) global threshold for the whole recording, and (ii) local threshold for each 30-s time window. In an exploratory approach, independent t-test analyses were conducted.
Results: Insomnia patients exhibited significantly higher EEG complexity during REM sleep than good sleep controls, across all channels (p = 0.03) and at the Fz electrode specifically (p = 0.009), regarding the locally thresholded Lempel-Ziv compression approach. Results were also significant in N1 across all channels (p = 0.04) and at the Fz electrode (p = 0.01), and a trend for significance was found in N2 at the Fz electrode (p = 0.05).
Conclusion: Although preliminary and exploratory, these findings support the view that increased EEG complexity in insomnia may reflect a state of hyperarousal possibly linked to cholinergic overdrive. In this model, REM sleep instability together with cortical hyperactivation during light sleep are thought to play a central role in driving sleep fragmentation and disturbances in emotional regulation. Overall, EEG complexity emerges as a potential marker of the dynamic interplay between stress, depression, and hyperarousal in insomnia. Complexity measures, by detecting subtle alterations in brain dynamics, could provide valuable insights from both clinical and neurophysiological perspectives, particularly in relation to the quality of restorative sleep and its impact on daytime cognitive and emotional functioning in ID. However, more detailed analyses are needed to further elucidate the finer characteristics of these brain dynamics.