Getting less than the recommended seven hours of sleep a day can nearly double the risk of a car crash, according to a new study from the AAA Foundation for Traffic Safety.

The American foundation analyzed 7,234 drivers involved in 4,571 crashes for their study.

They found that drivers who reported getting an hour or two less than the recommended seven hours of sleep in a 24-hour period doubled their risk for a crash.

"Our new research shows that a driver who has slept for less than five hours has a crash risk comparable to someone driving drunk," said Dr. David Yang, the executive director for the foundation, in a release.

Examining a 24-hour period, the AAA Foundation found that there was a steady climb in the risk of crashing with the less sleep you got.

Drivers who had:

  • six to seven hours of sleep had a crash risk 1.3 times higher than normal;
  • five to six hours of sleep had a 1.9 times higher than normal;
  • four to five hours of sleep had a crash risk 4.3 times higher than normal;
  • and those with less than four hours of sleep had a crash risk 11.5 times higher than normal.

Researchers say that while an overwhelming majority of drivers they interviewed said they viewed "drowsy driving" as dangerous behaviour, one in three admitted they drove while having a hard time keeping their eyes open at least once in the past month.

The foundation defines "drowsy driving" as having trouble keeping your eyes open, drifting from lanes or not remembering the past few kilometres you drove. Worryingly, more than half of drivers involved in crashes related to tiredness showed no symptoms before falling asleep at the wheel.

"Failing to maintain a healthy sleep schedule could mean putting yourself or others on the road at risk," said Jake Nelson, director of Traffic Safety Advocacy and Research for AAA.

The foundation recommends drivers should travel at times when they're normally awake, schedule a break roughly every 160 kilometres, take turns driving long distances with an alert traveller, and avoid medications that can cause drowsiness and high-caloric meals.