Study: This Is How Much Your Risk Of Causing A Car Crash Increases When You Don’t Get Enough Sleep


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Along with the many other areas of life affected by not getting enough sleep, inadequate shuteye also significantly increases the chances of causing a motor-vehicle crash. By how much? A new study took a shot at quantifying an answer to that question, and the results may surprise you.

Public health campaigns frequently remind us that driving drowsy can be just as dangerous as driving while intoxicated. But statistics suggest that even if most of us know that’s true, we aren’t doing enough to change it. Driver drowsiness is responsible for an estimated 7% of all motor vehicle crashes in the U.S. every year (that’s roughly 330,000 sleep-related accidents) and 16% of fatal crashes.

Despite recommendations that most adults need between seven and nine hours of sleep, surveys from the U.S. Department of Transportation (US DOT) and other agencies indicate that one in three adult drivers sleep fewer than seven hours a night, and many of us get less.

For this study, researchers reviewed and analyzed data from a previous study by the US DOT, which involved in-depth investigations from 5,470 crashes. The study data had the added dimension of including interviews with the drivers.

The results showed that compared to drivers getting seven to nine hours of sleep a night, those who reported getting six hours of sleep had 1.3 times the odds of causing a crash. Those who reported getting five hours had 1.9 times the odds, and those getting four hours had 2.9 times the odds.

Drivers who reported getting fewer than four hours had a startling 15.1 times the odds of causing a crash, which is comparable to the risk of a driver with a blood alcohol level 1.5 times the legal limit (that’s about nine drinks for an average-sized person).

Drivers getting four or fewer hours in the preceding 24-hour period also had the highest risk of single-vehicle crashes, which, according to the US DOT, are more likely to result in injury or death.

The study also found that driving for more than three hours without a break also increases risk, as do changes to a driver’s sleep schedule within the past week.

“Being awake isn’t the same as being alert. Falling asleep isn’t the only risk,” said lead study author Brian Tefft. “Even if they manage to stay awake, sleep-deprived drivers are still at increased risk of making mistakes–like failing to notice something important, or misjudging a gap in traffic–which can have tragic consequences.”

These results add to those of a study earlier this year that found strong correlations between inadequate sleep and prevalence of sleep apnea and motor-vehicle crashes. Severe sleep apnea (a condition resulting in obstructed breathing multiple times a night that affects an estimated 22 million Americans) was associated with 123% increased crash risk, and sleeping six hours a night, compared to seven or eight, was associated with 33% increased risk. Those results held true even for people who didn’t report feeling excessively sleepy during the day.

The results also jibe with those from a AAA Foundation report that found there’s a significant increase in crash risk for every hour of lost sleep. Drivers who slept five or six hours a night were twice as likely to crash as those who slept seven or eight hours, and those who slept only four hours were four times as likely to crash.

The latest study was published in the journal SLEEP.

You can find David DiSalvo on Twitter, FacebookGoogle Plus, and at his website,

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