During road trips, it’s not uncommon for drivers to look over and see passengers that have nodded off, their heads bouncing gently against the door window. Drivers sometimes find themselves fighting off drowsiness. And it’s not just long-haul truckers: Even an afternoon day trip can promote nodding off.

So why do car rides tend to turn us into bleary-eyed husks in need of caffeine or blaring music to remain alert?

According to Sleep.org, drowsy driving is often correlated with “highway hypnosis,” a term used for drivers and passengers who see their need to pay attention reduced when they’re on a predictable and monotonous route. For road trips, this can mean staying on a highway for extended periods without needing to check for exits or navigate the sometimes-tricky paths present on side roads.

The predictability promotes somnolence, or even a trance-like state. You might sometimes find yourself having covered a stretch of road without remembering much of it. And the longer the unchallenging drive, the sleepier or more unfocused drivers may become.

Road trips can also fall into a person’s natural circadian rhythms for increased drowsiness—typically in the early morning hours and mid-afternoons. A road trip that started out at 8 a.m., for example, and extends into 2 p.m. could see a vehicle’s occupants tired from both a routine highway circuit and the expected sleepiness they experience later in the day.

There’s also some limited research into the mechanics of a car affecting a person’s alertness level—specifically, how the vibrations of a moving car can relax the brain and body.

While a possible explanation, all we know for certain is that the lack of mental stimulus on a drive can promote fatigue. That can prove deadly, as the Centers for Disease Control (CDC) estimates up to 6000 fatal crashes a year are caused by drowsy or inattentive drivers. The U.S. National Highway Traffic Safety Administration estimates over 100,000 crashes a year involve people struggling to stay awake.

To stay safe, it’s best to take breaks, switch drivers, and make responsible use of caffeine to promote alertness. “Tricks” like turning up the radio or rolling down the window haven’t ever been demonstrated to have a real effect.

If you’re merely a sleepy passenger, your risks are few aside from a slightly sore neck. Enjoy the rest.

[h/t Sleep.org]