Drowsy drivers are a bigger risk factor in vehicle accidents than previously thought, reports ABC 7 Chicago.

And the problem gets worse when darkness falls. The research, carried out by the AAA Foundation for Traffic Safety, found that the “proportion of crashes in which drowsiness was evident was over three times as great during the nighttime hours,” according to Brian Tefft, a senior researcher at the foundation.

The researchers continuously monitored more than 3,500 people from six locations across the US for several months between October 2010 and December 2013, using in-vehicle cameras and other equipment. Of 701 crashes the researchers studied, drowsiness was a factor in 8.8 percent to 9.5 percent.