Epidemiological studies show a high prevalence of "insomnia" in adolescents. However, insomnia symptoms are not specific for insomnia disorder. Puberty is associated with circadian delay, which may cause insomnia symptoms such as problems falling asleep and daytime impairments, but also difficulties rising in the morning which is not a hallmark of insomnia disorder. The aim of this study was to investigate the extent to which adolescent insomnia symptoms may be attributed to circadian delay. The sample comprised 3,867 high-school-students. Survey instruments included the Bergen Insomnia Scale (BIS), the Munich ChronoType Questionnaire (MCTQ), the reduced Morningness-Eveningness Questionnaire (r-MEQ), and items on subjective sleep problems and sleep-related behaviors. Symptoms of circadian delay (CD) were defined as i) trouble waking on school days, ii) ability to sleep long into the day, iii) waking ≥10:00 on free days and/or iv) oversleeping for school. A total of 34.5% reported insomnia according to BIS. Among these, 88.4% reported ≥1 CD-symptom and 15.5% reported all CD-symptoms. Adolescents with insomnia and ≥1 CD-symptom were often evening-types (56.9%), had long social jetlag (2:55 h) and large school-/free day discrepancy in sleep duration (6:04 vs. 8:34 h). Results suggest circadian delay as a plausible explanation for insomnia symptoms in many adolescents.