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You’ve tossed and turned all night, and when the sun comes up, you get dressed, grab your coffee, and hop in the car. No big deal, right? But getting behind the wheel when you’re yawning could be more dangerous than you think, according to new research showing that drowsy driving can be as dangerous as driving drunk.

Whether you’re regularly pressed for sleep or you’re preparing for a long-haul drive for the holidays, you should know that even an hour or two of missed sleep can put you at greater risk for an accident on the road.

Drowsy driving is estimated to be a factor in 20 percent of fatal crashes. A new study from AAA states that drivers who skimp on the seven hours of sleep experts recommend increase their risk of a crash exponentially. Missing one to two hours of sleep doubles a driver’s crash risk, while foregoing two or three hours increases the risk of a crash by 400 percent, according to the AAA Foundation for Traffic Safety study. Those who sleep for less than four hours in a 24-hour period are 11.5 times likelier to crash, the study showed.

“Not getting enough sleep is extremely dangerous for drivers,” said Robert Sinclair Jr., manager of media relations for AAA Northeast. “Our new research shows that getting less than five hours sleep is the same as driving drunk.” Drivers who had slept for less than four hours, four to five hours, five to six hours, and six to seven hours in the past 24 hours were 11.5, 4.3, 1.9, and 1.3 times likelier to crash, respectively, than drivers who had slept for seven or more of the past 24 hours.

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The report also found that the crash rate for drivers who slept only four or five hours was “similar to the U.S. government’s estimates of the risk associated with driving with a blood alcohol concentration equal to or slightly above the legal limit for alcohol in the U.S.” The increase in crash rates associated with driving after sleeping for less than four hours is much greater. Meanwhile, a study from the UCLA Sleep Disorders Center found that a single beer can affect someone who slept only four hours the same way drinking six beers affects a well-rested person.

Tired drivers are more likely to be distracted, slower to react, and prone to dangerous decisions than well-rested drivers. Obvious signs of fatigue include frequent yawning, an inability to keep your eyes open, and drifting into the opposite lane or shoulder of the road. In some cases, the rumble strip on the side of the road vibrates to alert drivers when they’ve departed from the lane markers, but if you catch yourself nodding off, keeping too short of a following distance, or driving past your turn, you’ll know it’s time to take a break.

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However, more than half the drivers involved in fatigued-related crashes experienced no symptoms before falling asleep behind the wheel, according to the study. Many drivers who do recall nodding off are too embarrassed to report it. This makes it difficult to determine the role of fatigue in an accident. Also, most states don’t have a code on their crash report forms to indicate that a driver fell asleep. Nor is there a central database to track sleep deprivation as an underlying cause in a collision.

“Fatigue results in thousands of fatalities a year, and it’s underreported,” said Debbie Hersman, president and chief executive of the National Safety Council, an Illinois-based nonprofit that promotes health and safety. “People who have been awake for 15 to 18 hours, we really see their crash rates go up.”

Acute sleep deprivation is common, especially for night shift workers, students, doctors on-call, pilots, business travelers, and parents taking care of a sick child. Car accidents are the leading cause of workplace-related deaths, Hersman said. “People are burning the candle on both ends in our society. Work isn’t just eight hours a day anymore.”

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A survey by the Centers for Disease Control and Prevention found that 35 percent of adults usually sleep for less than seven hours daily and 12 percent of those report sleeping for five hours or less. Additionally, some prescription and over-the-counter drugs like cold medicine promote drowsiness as a side effect.

But knowing that sleep is critical to safety on the road isn’t necessarily enough to change drivers’ behavior. AAA found that 97 percent of drivers viewed drowsy driving as a safety threat, but nearly one-third of them admitted to having difficulty keeping their eyes open while driving during the previous month. Furthermore, many people don’t recognize when they’re drowsy. According to the UCLA Sleep Disorders Center: “People who are drowsy often don't even realize it. This makes being drowsy that much more dangerous. The effects of being drowsy are very much like the effects of drinking alcohol. This is because it is hard to know for sure when someone is too tired to drive. Drowsy driving is much more difficult to identify than drunk driving.” 

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Catching up on sleep isn’t as simple as squeezing in a catnap, the research shows. “If you are not getting enough sleep, then your body has a sleep debt,” according to the UCLA Sleep Disorders Center. “This debt continues to grow as you ‘add on’ more and more hours of missed sleep. As you miss more hours of sleep, you will find it harder to think and perform as well as you would like.”

How can you combat the risks of running on a sleep deficit? AAA recommends taking a break every two hours or 100 miles, traveling with a passenger who can keep you awake and help share the driving responsibilities, and avoiding medications that cause drowsiness. If all else fails and you find yourself dozing at the wheel, pull off the road and take a nap.