Insomnia becomes dangerous when it significantly impairs daytime function or signals a serious underlying condition. Red flags include dozing while driving, high accident risk, major cognitive impairment, severe depression or suicidal thoughts, manic symptoms (high energy with little sleep), perceptual disturbances, uncontrolled palpitations/chest pain, or shortness of breath. Untreated obstructive sleep apnea—often associated with snoring and breathing pauses—can raise cardiometabolic risk and should be assessed promptly when suspected. From a duration standpoint, insomnia ≥3 nights/week for ≥3 months suggests chronic insomnia and should trigger evidence-based care (CBT-I). Medications can be appropriate for selected cases but are typically considered within a careful, goal-driven framework. Prominent daytime sleepiness warrants evaluation for other sleep disorders (sleep apnea, circadian rhythm disorders, restless legs). Evidence base: ACP endorses CBT-I as first-line for chronic insomnia; NHLBI/AASM outline sleep apnea diagnosis and implications; reviews link insufficient sleep with cardiometabolic and cognitive harms.