According to a new study, people who have slept for fewer than seven hours have higher chances of getting involved in car crashes The risk is greatest for the drivers who have slept for fewer than four hours This is the first peer-reviewed study to quantify the relationship between how much a driver has slept and his or her risk of being responsible for a crash Researchers analysed data from a study conducted by US Department of Transportation that involves a sample of 5470 crashes (including interviews with the drivers) The research have found that drivers who have fewer than four hours of sleep had 15.1 times the odds of responsibility for car crashes Study author Brian Tefft said, Being awake isn't the same as being alert. Falling asleep isn't the only risk. Even if they manage to stay awake, sleep-deprived drivers are still at increased risk of making mistakes The detailed study is in the journal - Sleep