To investigate trend data in the prevalence of sleep-onset difficulties among Norwegian adolescents covering the age groups 11, 13 and 15 years. Data were based on the Health Behaviour in School-aged Children — A WHO Cross-National Survey (HBSC) — and were collected on six occasions between 1983 and 2005. At each point in time data were obtained from representative samples comprising between 3402 and 5026 adolescents. The prevalence of sleep-onset difficulties was higher among 11-year-old students compared to the 13- and 15-year-olds. Girls reported a higher prevalence than boys. A logistic regression analysis showed that the prevalence of sleep-onset difficulties overall had increased significantly since 1983, which constituted the reference year. When the analysis was broken down by age and gender, the same tendency was found in all groups. The prevalence of sleep-onset difficulties among adolescents has increased during the last decades. This development gives reason for concern and should receive more attention from teachers, parents and health professionals.