Driving while intoxicated gets most of the attention, but about one in every six fatal auto accidents in the U.S. is due to driving while drowsy, according to a new study by the AAA Foundation for Traffic Safety. Worse, it's something 41 percent of us admit to having done. "Just like alcohol and drugs, being very tired while you're driving decreases your awareness," says foundation president Peter Kissinger. "It slows your reaction time and it impairs your judgment." Here's a look at this "silent killer" responsible for thousands of deaths each year:

What counts as drowsy?

The AAA survey's criteria is that people reported being so tired they had trouble keeping their eyes open — 27 percent said they had driven like that in the past month. Among the 41 percent who said they either fell asleep while driving or momentarily nodded off, 11 percent said they had done so in the past year. Great, so "if that driver in the next car isn't speeding, intoxicated or distracted by texting or talking on the phone," say Jon Hilkevitch and Serena Maria Daniels in the Los Angeles Times, "there's a good chance he or she is barely awake."

Is this worse than expected?

The numbers are far higher than in previous estimates. According to this new study — which is based on National Highway Traffic Safety Administration (NHTSA) data from 1999 to 2008 — 16.5 percent of fatal crashes involved drowsy drivers, as did 13 percent of auto accidents that required hospitalization. Young drivers and men were the worst offenders.

How does drowsy driving compare to drunk driving?

About one in three deadly car crashes involve alcohol, says Deborah Kotz in U.S. News. So it's "ludicrous" that only New Jersey specifically forbids driving while sleep deprived, as defined by 24 straight hours without sleep. In the Garden State, any driver involved in a fatality who meets that criteria can be charged with vehicular homicide. Previous studies have shown that being awake for 24 hours can leave you with the equivalent of a 0.1 blood-alcohol level.

What are the warning signs?

Aside from the obvious ones — frequent yawning and heavy eyelids — drivers should watch out for daydreaming, missing road signs or exits, and feeling irritable and restless. But people aren't all that good at self-diagnosis: 70 percent of respondents said they felt they were awake enough to drive when they weren't. "People think that by rolling down the window or turning up the radio they will be able to offset drowsiness," says Dr. Thomas J. Balkin of the National Sleep Foundation. "But they lose touch... The process of actually falling asleep, we’re not good at identifying that."

Why not?

When your body is ready to sleep, it releases melatonin and other chemicals, says Dr. Robert Basner. When you force yourself to stay awake, your brain has to fight those chemicals as well as concentrate on the task at hand, leaving you in a fog.

Does coffee help?

It might, AAA says, if you give it or another caffeinated beverage about 30 minutes to kick in before you resume driving. The best deterrent, though, is traveling with a wide-awake passenger. Other tips from AAA include getting six hours of sleep before a road trip, driving at times when you are normally awake, and taking a break every two hours or 100 miles. Other solutions, says U.S. News' Kotz, include using a designated driver or taking public transportation.

Sources: ABC News, New York Times, Los Angeles Times, CNN, U.S. News