Story highlights Estimates indicate that about 1% to 2% of all accidents involve driving while drowsy

AAA researchers say the number was 8.8% to 9.5% for the crashes they studied

(CNN) Drowsy drivers are a bigger risk factor in vehicle accidents than previously thought, according to a study published Thursday.

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% to 9.5%.

Sleepiness was also a factor in 10.6% to 10.8% of crashes resulting in significant property damage, airbag deployment or injury.

The study says US government statistics indicate that only about 1% to 2% of all accidents involve driving while drowsy, but research has suggested that the problem might be much bigger.

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