A new study in SLEEP indicates that people who have slept for fewer than 7 of the past 24 hours have higher odds of being involved in and responsible for car crashes. The risk is greatest for drivers who have slept fewer than 4 hours.

Experts recommend that adults should sleep for 7 to 9 hours a night, yet government surveys indicate that 1 in 5 US adults sleeps for fewer than 7 hours on any given night, and 1 in 3 report usually sleeping for fewer than 7 hours. An estimated 7% of all motor vehicle crashes in the US and 16% of fatal crashes involve driver drowsiness.

While the dangers of driving drowsy were already well known, this peer-reviewed study quantifies the relationship between how much a driver has slept and his or her risk of being responsible for a crash. For this new study, researchers analyzed data from a previous study by the US Department of Transportation, which involved in-depth investigations of a sample of 5,470 crashes, including interviews with the drivers involved.

The researchers here found that drivers who reported fewer than 4 hours of sleep had 15.1 times the odds of responsibility for car crashes, compared with drivers who slept for the recommended 7 to 9 hours in the preceding 24-hour period, comparable to US Department of Transportation estimates of the crash risk of a driver with a blood alcohol concentration roughly 1.5 times the legal limit.

Researchers involved in the study also discovered that drivers who reported 6, 5, and 4 hours of sleep in the past 24 hours had 1.3, 1.9, and 2.9 times the odds of responsibility for a crash, respectively, compared with a driver who slept for 7 to 9 hours. Drivers who reported less than 4 hours of sleep had particularly elevated risk of single-vehicle crashes, which are more likely to result in injury or death. Drivers who had changed their sleep or work schedule in the past week and drivers who had been driving for 3 hours or longer without a break were also found to be at increased risk.

“Being awake isn’t the same as being alert. Falling asleep isn’t the only risk,” says study author Brian Tefft, in a release. “Even if they manage to stay awake, sleep-deprived drivers are still at increased risk of making mistakes—like failing to notice something important, or misjudging a gap in traffic—which can have tragic consequences.”