BACKGROUND: Insomnia is one of the most prevalent health concerns within the general population, with a multitude of existing electroencephalographic (EEG) spectral studies supporting heightened levels of high-frequency EEG activity as an index of cortical arousal. Nevertheless, traditional spectral analysis has been limited by its inability to distinguish between aperiodic and periodic elements. In contrast, a novel method, Spectral Parameterization (SpecParam), can separate these components and reveal the neural mechanisms of cortical arousal. METHODS: The aperiodic and periodic activities of 42 insomnia disorder (ID) patients and 45 age- and sex-matched healthy controls (HC) were evaluated during eyes-closed resting state. The associations between behavioral scales and aperiodic/periodic parameters were further examined to elucidate the underlying psychophysiological significance. RESULTS: We found that the aperiodic exponent was diminished in the ID group compared to the HC group. Additionally, the ID group exhibited an elevated central frequency and a more constrained bandwidth for periodic activity within the alpha band. Within-group correlation analyses revealed that a reduced exponent was associated with worse sleep quality and more frequent failures in inhibitory control within the ID group. CONCLUSIONS: A smaller exponent within the ID group may reflect impaired inhibitory neuronal activity, potentially leading to cortical hyperarousal. The association of a smaller exponent with deteriorated sleep quality and impaired inhibitory control supports this hypothesis. Furthermore, the increased center frequency of the alpha band across extensive brain regions, along with a narrower alpha bandwidth in the left frontal and right parieto-occipital regions, may represent abnormal manifestations associated with excessive arousal. In summary, these results support the role of aperiodic activity as an index of impaired excitation/inhibition balance in neural activity within in ID group.