Sleep deprivation is a known risk factor for car crashes. In fact, researchers estimate that drowsy driving is responsible for 20 percent of all car crashes in the United States. But, up until now, studies have not focused on young drivers. Researchers point out that young people should be a target of education efforts to eliminate drowsy driving because their alertness, mood and physical performance are more adversely affected by sleep deprivation than older, more experienced drivers with similar sleep deprivation.

Sleep deprivation contributes to car crashes because it impairs elements of human performance that are critical to safe driving. Sleepiness reduces optimum reaction times, causes delayed responding in attention-based tasks, and increases the time it takes to process and integrate information. Even moderately sleepy drivers can have a performance-impairing increase in reaction time that will prohibit stopping in time to avoid a collision.

The new study involved more than 19,000 young, newly-licensed drivers living in New South Wales, Australia, who fielded questions about their sleep habits, including weeknights and weekends. Then researchers tracked the participants, aged 17-24, for two years, and obtained police reports to document car accidents.

The drivers who reported sleeping six or fewer hours per night were about 20 percent more likely to be involved in a car crash, compared with those who got more than six hours of sleep a night. Among the sleep-deprived, car crashes were most likely to occur between 8 p.m. and 6 a.m.

Even after researchers considered other factors that affect people’s risk of a crash, such as age, number of driving hours per week, risky driving behavior such as speeding and a history of car accidents, the same findings held.

The researchers did note however, that participants were only asked about their sleep habits once over the course of the study, so the exact number of hours participants slept the day before they were involved in a crash is unknown.

Still, researchers hope the new findings will increase understanding of the impact of reduced hours of sleep on crash risk, and pinpoint subgroups of young drivers to facilitate education.