Insomnia disorder is one of the most common sleep disorders and is traditionally conceptualized in terms of hyperarousal, altered sleep homeostasis, and cognitive-behavioral perpetuating factors. The two-process model of sleep regulation indicates that sleep initiation and maintenance depend not only on homeostatic sleep pressure but also on circadian phase, amplitude, and stability. Evidence suggests that some patients presenting with insomnia complaints have misalignment between endogenous circadian phase and the intended sleep window, whereas others show reduced rest-activity amplitude, increased sleep-timing variability, or abnormal light exposure patterns. This narrative review integrates clinical, measurement, mechanistic, and intervention evidence on circadian dimensions in insomnia disorder and proposes a candidate circadian phenotyping framework for future validation. The framework combines sleep diaries, actigraphy, light exposure assessment, dim-light melatonin onset (DLMO), and core body temperature rhythms to distinguish circadian rhythm sleep-wake disorder (CRSWD)-dominant insomnia complaints, insomnia disorder with circadian modifiers, and comorbid insomnia disorder and CRSWD. Cognitive behavioral therapy for insomnia (CBT-I) remains first-line treatment. Fixed wake time, morning light, evening light restriction, timed low-dose melatonin, scheduled activity, and multicomponent approaches are positioned as candidate adjunctive modules to be tested in phenotype-stratified trials. These phenotypes are not diagnostic categories, clinical decision algorithms, or treatment guidelines. The review instead translates circadian evidence in insomnia into testable propositions, including provisional phenotype definitions, recommended measurement strategies, falsifiable predictions, and future trial designs. Operational thresholds, reproducibility, predictive validity, incremental treatment benefit, optimal parameters, safety, implementation feasibility, and cost-effectiveness require prospective phenotype-stratified randomized trials and long-term follow-up.
