Driving under the influence of alcohol or drugs is known to increase the chances of causing an accident. For instance, marijuana can impair drivers' reaction time. But what about drowsiness? As many as a third of all fatal car crashes might involve fatigued drivers, according to research by the National Highway Traffic Safety Administration.

And a new study finds that driving while under the influence—of drowsiness—is exceedingly common.

More than one in 25 people report actually having fallen asleep behind the wheel at least once within the past month, according to a new study from the U.S. Centers for Disease Control and Prevention (CDC). Unsurprisingly, drivers who are at risk of dozing are more likely to cause crashes that result in injuries or death than are alert drivers. "Drowsiness slows reaction time, makes drivers less attentive and impairs decision-making skills," the report authors noted.

The researchers, led by Anne Wheaton, an epidemiologist at the CDC, analyzed responses from more than 147,000 adults who participated in the Behavioral Risk Factor Surveillance System phone survey. Respondents hailed from 19 states and the District of Columbia; within these areas, Texas—with its 3,200-plus miles of interstates–had the highest rate of severely tired drivers, with 6.1 percent of respondents saying they had slumbered at the wheel, and Oregon had the lowest, with just 2.5 percent.

Not surprisingly, people who reported having zonked out while driving were more likely to say they most often got six or fewer hours of shuteye and/or snored, which can be a sign of sleep apnea. Older drivers—those 65 and up—were the least likely to report having caught some winks while driving (just 1.7 percent had), whereas those ages 25 to 34 and 35 to 44 were the most likely (6.3 percent and 5.5 percent, respectively). Those in these age categories might be more likely to be working shifts or slogging through long commutes, while those who are retired were by far the least likely to have reported falling asleep (1 percent), suggesting they might be getting ample rest and not feel compelled to drive when they are not up for it.

These numbers are likely to be below the actual rate of somnolent automobilists because they rely on self-reported responses; many people who nod off for just a second or so don't even realize it has happened. Additionally, these stats represent those who have actually dozed at the wheel; more than a quarter of adults report driving while they feel bushed in a given month, according to a recent poll by the National Sleep Foundation.

Those most at risk for being severely sleepy at the wheel included commercial drivers, people with sleep disorders who are not being properly treated, people who take sedatives, people who generally do not get enough sleep, and people who work long or night shifts.

Shift workers often operate short on sleep. Research published last year by the CDC also showed that those working in dangerous industries—including transportation jobs such as commercial driving. In fact, that study found that nearly 70 percent of people who worked overnight shifts in transportation or warehousing industries often got fewer than six hours of z's.

Most fatigue-induced accidents occur in the afternoon and nighttime. The best thing to do to avoid dangerous drowsiness is to pull over and rest. Common "techniques to stay awake while driving, such as turning up the radio, opening the window, and turning up the air conditioner, have not been found to be effective," the authors of the new CDC study reported. It remains to be seen whether new car technology to sense and alert drowsy drivers can help. In the meantime, hopefully the road safety worries aren't enough to cause any more lost sleep.