The aim of the present study was to investigate the prevalence of "behaviorally induced insufficient sleep syndrome (BIISS)" which is a newly defined hypersomnia, among adolescents. BIISS is characterized by excessive daytime sleepiness, short habitual sleep duration and sleeping considerably longer than usual during weekend/vacations. The study was conducted in the Hordaland County, Norway using a cluster sampling procedure. In all, 1285 high school students (aged 16-19 years) participated by completing self-report questionnaires on a computer. The estimated prevalence of BIISS was 10.4%. The results from logistic regression analyses showed that use of alcohol and living in an urban area were positively related to BIISS, whereas a high level of education in mothers was negatively related to BIISS. BIISS was associated with poor grades and symptoms of anxiety and depression. (Contains 2 tables.)