More than 40 percent of U.S. drivers admit to having fallen asleep at the wheel, according to a study just released by the AAA Foundation for Traffic Safety.

The study calls attention to one of the leading causes of fatal crashes -- driving  while drowsy. National Highway Traffic Safety Administration figures indicate that one in six fatal crashes involve a drowsy driver, AAA says.

The AAA study found that one in 10 motorists surveyed said they had fallen asleep at the wheel sometime in the past year. Over the  course of their driving careers, 41 percent reported an incident of falling asleep.

The study found  that one-quarter of drivers admitted to driving while having difficulty keeping their eyes open at some point in the past month. Despite that, 85 percent said it was "completely  unacceptable" to drive while struggling to remain awake.

AAA recommends  that drivers get at least six hours of sleep before a long trip and take breaks every two hours  or 100 miles. The group also urges drivers to travel at times when they are used to being awake and to make an overnight stay rather that attempting to drive to a faraway destination  without resting.

According to AAA, the problem of sleep deprivation while driving becomes especially common with the end of daylight savings time because of the earlier onset of  darkness.