Insomnia is increasingly understood as a heterogeneous disturbance of sleep-wake regulation shaped by interacting cognitive, physiological, circadian, neurobiological, and perceptual mechanisms. This chapter develops a phenotype-informed framework for insomnia that distinguishes descriptive clinical presentations from underlying mechanistic dimensions and measurable biomarker domains. Core phenotypes reviewed include cognitive-emotional hyperarousal, objective short sleep duration, paradoxical insomnia and sleep-state misperception, circadian misalignment, autonomic dysregulation, sleep-depth instability, psychiatric comorbidity, and night-to-night sleep variability. Particular attention is given to neurocognitive biomarkers, including attention, executive function, working memory, episodic memory, cognitive variability, and subjective cognitive burden, with emphasis on how outcomes differ across insomnia phenotypes. It also examines electrophysiological and sleep microstructure markers, including high-frequency EEG activity, slow-wave activity, spindles, microarousals, cyclic alternating pattern, cardiopulmonary coupling, and Odds Ratio Product as emerging indicators of sleep depth and state instability. Autonomic, circadian, physiological, neuroimaging, wearable, and AI-enabled multimodal approaches are evaluated for their potential to refine mechanistic understanding and support future precision sleep medicine. Across domains, the chapter emphasizes that no single biomarker currently provides sufficient specificity for diagnosis or treatment selection. Instead, insomnia phenotyping is most useful as an integrative, mechanism-informed approach that can improve clinical formulation, guide individualized treatment emphasis, and identify future research priorities.
