Memorial Day means the start of the annual summer driving season. Americans hitting the highways should be aware of a major road hazard: sleepy drivers. A study by researchers at Stanford University Medical Centre indicates that sleep impacts reaction time and driving performance as much as alcohol, and that sleep-deprived drivers pose a major risk on the highways.

"Our study demonstrates that driving while sleepy is at least as dangerous, if not more dangerous, than driving while intoxicated," said Nelson B. Powell, MD, codirector of the Stanford Sleep Disorders Clinic and Research Centre.

The results of the study, co-authored by Powell and researchers from Washington University School of Medicine in St. Louis, were published in the May issue of Laryngoscope.  The study was conducted in collaboration with General Motors Corp and NBC News.

A large percentage of Americans suffer from acute or chronic sleep deprivation and experience daytime sleepiness. While sleep loss can lead to errors in daytime activities, Powell said the magnitude of the risks are not widely acknowledged or understood. Powell's team conducted the study to emphasise the effects of sleep deprivation on a common daytime activity - driving - and to raise awareness of the serious consequences associated with a lack of sleep.

Researchers studied 16 healthy, adult volunteers to ascertain changes in the reaction times and driving performance of the volunteers when they were either sleep-deprived or alcohol-impaired. During the first round of the study, volunteers drove a 1.4-mile performance course at GM's proving grounds in Arizona while in a fully rested, non-sleepy state. The drivers encountered several hazards and were judged in part by their reaction to the hazards.

After the initial round, the volunteers were split into three groups to repeat the tests. The first group was tested after spending one night without sleep, the second after sleeping for no more than two hours for seven consecutive nights and the third after consuming alcohol. The mean blood-alcohol level of the third group was 0.089 percent, which is slightly above the legal limit of 0.08 percent in California and most states.

As expected, researchers found that reaction times and driving performance significantly diminished during the second set of tests. For example, members of alcohol-impaired group on average took an additional 61 milliseconds to react. 
More surprising, researchers found little difference in reaction time and driving performance among the sleep-deprived groups and the alcohol-impaired group. Powell noted that the average reaction time of the alcohol-impaired group was 294 milliseconds, while the average for the sleep-deprived groups was 300 milliseconds.

Powell and the research team hope the results of the study promote public dialogue and additional studies about the dangers of sleepy driving. "These results should raise awareness about the seriousness of sleep deprivation," he said. "It's crucial that people recognise the danger of driving while sleepy and that people stay away from their cars while they're tired."