In the US alone, more than 6,000 people die in drowsy driving-related motor vehicle crashes each year, a study suggests.
Millions of US drivers fall asleep at the wheel each month, and roughly 15 per cent of all fatal crashes involve a drowsy driver, researchers note in the journal Sleep.
"Drowsy driving is not just falling asleep at the wheel; it mimics alcohol-impaired driving in many ways," said lead study author Stephen Higgins, a researcher at the National Highway Traffic Safety Administration in Washington, D.C.
"Drowsiness leads to slower reaction times and impaired attention, mental processing, judgment, and decision making," Higgins added by email.
Getting enough sleep every night is the best defense against drowsy driving, Higgins and colleagues note.
When that's not possible, a nap to break up the road trip may still help drivers remain more alert behind the wheel.
If you start to get sleepy while you're driving, pull over for a short 20 to 30 minute nap in a safe place
"If you start to get sleepy while you're driving, pull over for a short 20 to 30 minute nap in a safe place, such as a lighted designated rest stop," Higgins said. "You can also combine a caffeinated drink with the nap - this has been shown to increase alertness in scientific studies but only for short time periods."
For the study, Higgins and colleagues examined data from previously published research on drowsy driving to identify the main causes of the problem and potential ways to prevent it.
Many lifestyle factors can influence the odds of drowsy driving, they note. These include working long and irregular hours, working night shifts and having multiple jobs.
In addition, people who tend to frequently visit nightclubs or who are thrill-seekers also are more likely to be drowsy drivers.
People who don't fit the profile of a typical drowsy driver can still be at increased risk for nodding off behind the wheel at some point due to life events, the authors also note. For example, people who don't get enough rest the night before embarking on a long family road trip might be drowsy drivers.
To calculate the toll of drowsy driving, researchers examined crash data from 2010.
They found 32,999 total fatal crashes and 3.9 million total injury crashes.
Drowsy driving accounted for about 5,445 fatal crashes and 510,900 non-fatal collisions, for a total estimated societal cost of $109 billion per year, based on the 2010 figures.
Too often, people drive when they're tired because they have no choice, said Russell Griffin, a researcher at the University of Alabama at Birmingham who wasn't involved in the study.
"It would be great if everyone had the ability to carpool or to had access to public transportation, but without these options all they can do is drive themselves regardless of how tired they may be," Griffin said by email.
"Another reason people drive while tired is that they underestimate the risks of driving drowsy," Griffin added. "If we are able to educate the public of the dangers of drowsy driving, this may drive people to either find alternate modes of transportation when tired or plan ahead when they know that they will have to be driving while tired (e.g., driving home from work)."
While insufficient sleep is most often the cause of drowsy driving, other health issues can sometimes contribute to the problem, said Dr. Donald Redelmeier, a researcher at the University of Toronto who wasn't involved in the study.
"Sometimes drowsy driving is caused by a medical disorder such as sleep apnea, narcolepsy or a drug side effect," Redelmeier said by email.
Beyond taking precautions to remain awake and alert behind the wheel, drivers should also keep other common-sense safety measures in mind.
"Being drowsy is one of many factors that can lead to driver error and a life-threatening traffic crash," Redelmeier said. "At a minimum, be sure to buckle your seatbelt, respect the speed limit, avoid using cell phones, signal your turn, yield right-of-way, and do not drink and drive."