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Cars: Your ride to an early grave? You know accidents are a big risk when you climb behind the wheel. But a new study from Australia also links driving to obesity, poor sleep, stress, and other life-shortening health issues.

The Aussie study team asked roughly 37,000 people to answer questions about their daily drive times, sleep schedules, exercise routines, and a handful of other health factors. Compared to non-drivers, people who spent two hours (or more) on the road every day were:

  • 78 percent more likely to be obese
  • 86 percent more likely to sleep poorly (less than seven hours)
  • 33 percent more likely to report feeling psychologically distressed
  • 43 percent more likely to say their quality of life was poor

Regular road warriors were also a lot more likely to smoke and fall short of weekly exercise targets, the study data shows.

But don't get stuck on the two-hour threshold; even 30 minutes of daily drive time increases your risk for all of these negative health issues, the research shows.

So what's so bad about driving? "At this point, we can only speculate," says study coauthor Melody Ding, Ph.D., a research fellow at the University of Sydney. But here are her three best guesses, which, alone or in combination, could explain how driving hurts your health. And know this:

1. Sitting a lot is bad for you. "Especially uninterrupted sitting where you're not standing up for long periods," Ding says. There's some evidence that sitting hurts your body's ability to burn fat, which may explain its attendant health risks. Ding says some scientists even believe sitting for long stretches shortens your life regardless of your physical activity levels (although that's still being hotly debated).

2. Driving is stressful. Study after study links stress to cancer, heart disease, and a lot of other scary health issues. And researchers have found driving is one of the most stressful activities people do on a daily basis. "Driving-related stress could explain some of the mental health risks we observed," Ding adds. Research suggests managing stress could help offset some of driving's health risk.

3. Road time is lost time. There are only 24 hours in a day. And if you're spending a couple of them on the road, you may not have time left over for exercise, sleep, cooking healthy meals, and other beneficial behaviors, Ding says. Public transportation might also be a safer option because it involves more walking and standing than driving, she adds.