Importance Short sleep duration is common in adolescents and young adults, and short sleep duration is a risk factor for motor vehicle crash. OBJECTIVE To assess the association between hours of sleep and the risk for motor vehicle crash, including the time of day of crash and types of crash (single, multiple vehicle, run off road, and intersection). DESIGN Prospective cohort study. SETTING New SouthWales, Australia. PARTICIPANTS Questionnaire responses were obtained from 20 822 newly licensed drivers aged 17 to 24 years. Participants held a first-stage provisional license between June 2003 and December 2004 prospectively linked to licensing and police-reported crash data, with an average of 2 years of follow-up. Analyses were conducted on a subsample of 19 327 participants for which there was full information. EXPOSURE Sleeping 6 or fewer hours per night. MAIN OUTCOMES AND MEASURES The main outcome variablewas police-reported crash. Multivariable Poisson regression models were used to investigate the role of sleep duration on the risk for crash. RESULTS On average, those who reported sleeping 6 or fewer hours per night had an increased risk for crash compared with those who reported sleeping more than 6 hours (relative risk [RR], 1.21; 95%CI, 1.04-1.41). Less weekend sleep was significantly associated with an increased risk for run-off-road crashes (RR, 1.55; 95%CI, 1.21-2.00). Crashes for individuals who had less sleep per night (on average and on weekends) were significantly more likely to occur between 8 PM and 6 AM (RR, 1.86; 95%CI, 1.11-3.13, for midnight to 5:59 AM and RR, 1.66; 95%CI, 1.15-2.39, for 8:00 PM to 11:59 PM). CONCLUSIONS AND RELEVANCE Less sleep per night significantly increased the risk for crash for young drivers. Less sleep on weekend nights increased the risk for run-off-road crashes and crashes occurring in the late-night hours. This provides rationale for governments and health care providers to address sleep-related crashes among young drivers.