Is 6 hours of sleep enough for driving?

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 behavior. 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 an earlier 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.

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“We want people to understand how dangerous it is to drive drowsy,” Short told NBC News BETTER, especially during the summer when kids are out of school and people are more likely to take long road trips. “Drowsy driving causes a few things: one, you’re going to have a delayed reaction, and that is critical when you’re driving ... to be able to respond if you have an unexpected event. The second thing is that your decision making is impaired.”

In fact, The National Sleep Foundation reports that sleep deprivation can have similar effects on your body as drinking alcohol. "Being awake for 18 hours straight makes you drive like you have a blood alcohol level of .05 (for reference, .08 is considered drunk)," they report. "If you’ve been awake for a full 24 hours and drive — say, after a night where you just couldn’t fall asleep — it’s like you have a blood alcohol level of .10."

To help spread awareness about the dangerous effects that fatigue can have on drivers, Short is travelling to a few cities this summer conducting drowsy driving simulations. During the simulation, participants don a 23-pound suit and goggles that replicate how your body feels when you are driving in a drowsy state, including delayed blinking patterns, sluggishness and extreme tiredness.

“The simulation goggles change the way your eyes close and really give you the idea of what it’s like to drive while drowsy,” says Short. “As you get tired, the way we can tell is by your percent of eye closure, so every 10 seconds, the goggles close for one second; this represents being a medium level of drowsy, and mimics your eye pattern when you’re tired. If you’re truly drowsy and you nod off, it can be 2-4 seconds of eye closure at a time.”

The weighted suit works to slow your reaction time, similar to how it is slowed when you're tired. "When that reaction is slowed, you don’t always feel the effects," says Short. "Unfortunately, drowsiness doesn’t just happen quickly; it happens over time.”

I suited up and took part in the simulation when Short set up shop in the parking lot of Citi Field in New York city earlier this summer. It was definitely unnerving (to say the least) to have my field of vision obstructed every 10 — and then every 4 — seconds. When told to turn either left or right at the last minute, having a reduced field of vision slowed my decision making skills — and then I had a delayed reaction time once I did decide which way to turn, thanks to the weights on my ankles and wrists.

While I attempted to navigate the course, Short reminded me that while we were driving on a closed course going just 15-25 miles per hour, on the highway, you’re driving 60 to 70 miles per hour and covering a lot more area, which really increases the risk.

4 signs that you’re too tired to be behind the wheel

Reducing the occurrence of driving drowsy starts with being able to recognize the signs that you may be too tired to be behind the wheel. Short says to look out for these commons signals that our bodies are fatigued and our ability to drive may be impaired:

• Yawning. Continually yawning or the inability to keep your eyes open. Yawning is one of the simplest indicators to signal that you're tired, Short says.
• Memory lapse. “If you don’t remember the last couple of exits or last couple of miles, you’re getting tired and need to stop,” she says.

• Lane deviation. If you’re starting to go back and forth in the lane, drifting from lane to lane and/or engaging the rumble strips on the side of the road, it's time to pullover.
• Unsafe driving maneuvers. Frequent tailgating, braking or other unsafe driving maneuvers are a sign you may be dozing off, especially if active safety features (like lane keep assist, lane departure warning and others) have been engaged more than average.

Strategies to help you get home safe

We’ve all tried to power through that last 30 minutes of a long journey or had too early of a start in the morning when we're still groggy. Short encourages us to recognize when we're too tired to drive and not get behind the wheel in the first place. When you do recognize the symptoms above, "number one is always to get off the road,” says Short. But until you can, there are some strategies you can use to help keep yourself alert.

1. Call someone. “Call a friend, call your mom, or if you are in a vehicle with other passengers wake them up and have them talk to you, and really get your mind engaged," says Short.
2. Take advantage of the safety systems in your car. Many cars come equipped with features designed to keep you safe on the road. “Turn on your active safety systems," says Short. "At Chevy, we have lane departure warning and lane keep assist, those will help you stay in the lane. And forward collision alert. Give yourself enough time as possible to make a decision."
3. Play mental games. Anything that gets your brain engaged mentally will help you stay alert. "If you start to feel your mind wander, go old-school and play eye-spy, 20 questions or the billboard alphabet game," says Short. "Engaging your mind can often help you stay alert, so you can focus on the road ahead."
4. Consider a nap. AAA says to not underestimate the power of a quick nap. If you’re on a longer drive or road trip, pulling into a rest stop and taking a quick catnap — at least 20 minutes and no more than 30 minutes of sleep — can help to keep you alert on the road, they say.
5. Don’t rely on quick fixes. Short stresses not to rely on energy drinks or coffee to power you through your trip. There is no better way to tackle tiredness than getting the recommended amount of sleep, she says. If you’re too tired to focus, consider pulling over in a safe place for a nap or find the nearest hotel to get a good night’s rest, she says. “Don’t be fooled, the only antidote for drowsiness is sleep,” said William Van Tassel, manager of Driver Training for AAA. “Short term tactics like drinking coffee, singing, or rolling down the window will not work. Your body’s need for sleep will eventually override your brain’s attempts to stay awake.”

READY TO GET MORE ZZZ’S? READ THESE AND REST BETTER

• 7 Ways to Actually Get to Bed An Hour Earlier Tonight
• Why Lack of Sleep Is Costing Us Billions of Dollars
• 8 Sleep Mistakes You Can Fix Tonight
• 16 Ways to Get a Better Night’s Sleep Without Popping a Pill

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