Background: Insomnia is the most prevalent sleep disorder, yet it is often dismissed as benign. This review examines insomnia as an “economic disease” whose phenotype, comorbidity burden, treatment responsiveness, and societal costs are closely connected. Methods: This narrative review integrated five evidence streams: diagnostic and epidemiological data; pathophysiological models (hyperarousal and the predisposing–precipitating–perpetuating framework); prospective and meta-analytic evidence linking insomnia to depression, anxiety, suicide, hypertension, cardiovascular disease, type 2 diabetes, and dementia; randomized evidence on cognitive behavioral therapy and pharmacotherapy; and health-economic studies of costs and cost-effectiveness. Large cohorts, meta-analyses, guidelines, and cost-of-illness studies were prioritized. Results: Insomnia disorder affects approximately 10–20% of adults and is usually chronic. Insomnia precedes or aggravates costly conditions: it roughly doubles the risk of incident depression, is associated with an approximately 1.5- to 3-fold higher risk of suicidal thoughts and behaviors, and is linked to higher risks of hypertension, cardiovascular events, type 2 diabetes, and dementia. Total annual costs in the United States have been estimated at up to approximately $100 billion, driven mainly by absenteeism and reduced on-the-job productivity. Cognitive behavioral therapy for insomnia is effective and cost-effective, whereas hypnotics carry risks of falls, fractures, and cognitive impairment that add further costs, particularly in older adults. Conclusions: Insomnia is an economically consequential and frequently untreated chronic condition. Wider and earlier access to evidence-based non-pharmacological treatment is the strongest available lever for reducing its costs. Treating insomnia as a modifiable risk factor, rather than an isolated symptom, represents a high-value public health investment.