The objective is to examine incidence, prevalence, and persistence of restricted or short sleep among adolescents and associated risk factors. Data are from a sample of 4,175 youths aged 11 to 17 at baseline and 3,134 followed up 1 year later. Restricted sleep was defined as 6 hr or less per night. Prevalence was 20%, 1-year incidence was 17%, and chronicity was 54%. Analyses identified few independent predictors: female, older age, schoolwork, extracurricular activities, and life stress. Odds ratios for incidence of sleep restriction on weeknights were 10 for 8 or more factors and 17 for persistence with 8 or more risk factors. This prospective study documents that incidence of sleep restriction was high. Restricted sleep also was prevalent and chronic and, like many other health problems, has multifactorial origins. Having multiple risk factors substantially increased risk of restricted sleep. Given the importance of sleep in this population, more focus is clearly needed, both clinically and epidemiologically.