A new study about tired drivers and crashes has yielded surprising results.

In the past, experts attributed about 2 percent of car crashes to driver fatigue. It now appears that figure is off—by quite a bit. The new study claims driver fatigue is the culprit in 20 percent of crashes. That’s 10 times as many!

The driving study, by the Virginia Tech Transportation Institute, also found more fatigue-related crashes happen during the day than at night—the opposite of what one might commonly believe.

Researchers identified drivers’ behavior that was far beyond yawning, including eyelid closure, head bobbing, and severe loss of facial musculature—all indicators that drivers had actually fallen asleep.

The group responsible for the most fatigue-related crashes, according to researchers, was 18- to 20-year-olds.

If you’re sleepy behind the wheel, the safest thing to do is stop driving immediately—either pull over to rest, or trade driving time with one of your passengers.