For the study, researchers under the direction of Nicholas Moore at the Centre Hospitalo-Universitaire de Bordeaux analysed information from 679 drivers admitted to a hospital in southwestern France for more than 24 hours because of a serious accident between 2007 and 2009.

"Sleepiness carried almost as much risk as alcohol ingestion," wrote Moore and his colleagues in a letter in the Archives of Internal Medicine.

The researchers used information from driver questionnaires and police reports to determine what may have contributed to the accidents. Drivers reported what medications they were on, their alcohol use and how sleepy they had been before the crash, while patient files provided information on blood alcohol levels.

The majority of the injured drivers were under 55 years old and men. Over half were on a motorcycle, about one-third in a car and 10 percent on a bicycle at the time of the accident.

The police determined that 355 of the drivers were responsible for their crash. From that, Moore and his colleagues found that being between the ages of 18 and 29 years old, driving a car, drinking alcohol and being sleepy were all tied to an increased risk of causing an accident.

"We know from experimental studies that just four hours of sleep loss will produce as much impairment as a six pack (of beer)," said Christopher Drake, an associate scientist at the Henry Ford Hospital Sleep Disorders and Research Centre in Detroit.

"If you have a whole night of sleep loss, that's equivalent to having a blood alcohol content of 0.19," said Drake, who was not involved in the study.

A blood alcohol level of 0.08 percent is considered legally drunk in most US states.