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If you're driving and missing 2 hours of sleep, it could be as dangerous as driving drunkby Mike Denison

You probably already know you shouldn't drive when you're sleepy. But an AAA Foundation study released Tuesday showed just how dangerous it can be.  

Driving while missing two hours of sleep nearly doubles the chance of getting into an accident -- that's about as dangerous as driving while legally drunk, according to the study. Drive on less than four hours of sleep, and you're 11.5 times more likely to crash than a well-rested driver.

The study used crash data from more than 4,500 crashes and 7,200 drivers from 2005 to 2007.

'Not fit to operate a motor vehicle'

Past research showed that 7 percent of all U.S. crashes and 21 percent of deadly crashes involved a sleepy driver. 

Experts recommend that adults get at least seven hours of sleep per day. Drivers with four or five hours of sleep were considered just as likely to crash as someone with a blood-alcohol concentration over the legal limit. If you're coming out of an all-nighter, you're "not fit to operate a motor vehicle," according to the National Sleep Foundation.

This isn't the first study to show driving drowsy is dangerous, but it is one of the first to quantify just how much sleep you need. Prosecutors said this bus crash in New York from 2011, pictured above, was caused by a sleepy driver.

"You cannot miss sleep and still expect to be able to safely function behind the wheel."

David Yang, AAA Foundation for Traffic Safety

Aside from the obvious advice to sleep more, the AAA Foundation recommends avoiding heavy food, taking breaks every two hours (or 100 miles) and having an alert passenger on road trips. 

This study also didn't have data for crashes between midnight and 6 a.m., so it may actually be underestimating the danger.

This might not be a bad call.