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If you've ever been on a long road trip, you may have gotten an hour or so in only to notice you're feeling kinda... relaxed. Tired, even.

That's not just a problem for getting to your destination on time. According to the UK's Royal Society for the Prevention of Accidents, driver fatigue may contribute to around 20 percent of road accidents. Of course, not catching all the right Z's the night before is one thing, but nodding off in cars seems to happen even if you're healthy and rested.

In other words, a lot of us are prone to this effect. And we actually don't totally understand why. One idea is that white noise makes us sleepy.

In this case, the low drone of tires passing over the road. And there have been a few small studies to test that idea. For example, in a 2015 study, nineteen participants were asked to drive on simulated roads both quiet and loud.

And when they drove on the loud road, they showed more signs of fatigue — like driving more slowly or accidentally crossing lanes. But there are a few snags. For one thing, people didn't actually say they felt more tired, so it's really hard to tell whether noise was really the causal factor.

Instead it might be the vibration of the car, rather than the road noise, that makes us drowsy. That's the conclusion of a 2018 study where fifteen participants sat in a driving simulator that had been rigged up to a vibration table. Participants were asked to “drive” for an hour at high speed while the platform either vibrated four to seven times per second, or was still.

They rated how sleepy they felt before and after driving, and the researchers monitored their heart rate as a measure of drowsiness. After pretend driving for an hour, those who had been vibrating said they felt really sleepy, whereas those who got the smooth ride didn't. And it didn't even take that full hour for those in the vibrating group to start showing signs of fatigue.

After just fifteen minutes of driving, those participants' heart rate patterns indicated they were drowsy. Now I'm thinking that we need somebody to shake my bed! What the researchers think was happening was that the vibration was activating drivers' parasympathetic nervous system which, generally speaking, slows and relaxes the body.

What's kind of strange was that the heart rate pattern researchers saw was actually a sign of sympathetic nervous system activation. Which, again broadly speaking, does the opposite. It's responsible for the fight or flight response, for example.

The researchers concluded that this sympathetic activation was a sign of the body trying to compensate for the drowsiness brought on by the vibration. In other words, the vibration is making a person sleepy, making it harder for them to drive, so the sympathetic system kicks in to help them concentrate, which shows up in the form of changes to their heart rate. Maybe cars of the future will have some extra shock absorption to minimize all that jigglin' so I won't be so sleepy.

But until then, at least you know to look out for rough roads if you want to avoid carcolepsy. Also, if you're feeling sleepy at all, go… pull off, get some Wendy's or something. Don't take the risk.

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