Here's a sobering thought: According to the National Sleep Foundation (NSF), "Driving while drowsy is no different than driving under the influence of alcohol or drugs." That's because sleepiness has the same effect: your reaction time slows, your awareness decreases, and your judgment is impaired. The U.S. National Highway Traffic Safety Administration estimates that 100,000 police a year crashes report crashes that involve drowsiness and fatigue as a principal factor. At least 71,000 people are injured and 1,500 killed in sleep-related crashes each year.

In a recent NSF poll, 51% of adults admitted to driving while drowsy in the past year, and 24% of those 18-29 years old reported they had actually fallen asleep at the wheel. To avoid being one of them, here are the NSF's warning signs of driver fatigue:

You can't remember the last few miles driven

You drift from your lane, keep jerking your car back into the lane, or hit a "rumble" strip on the shoulder

You have difficulty focusing, keeping your eyes open, or your head up

You tailgate or miss traffic signs

Fred Mottola suggests that one way to avoid drowsy driving is to become aware of your personal biological clock. What times of day do you feel most tired? Almost everyone feels sleepy mid-afternoon. If you're already on the road and realize that you are susceptible to falling asleep, you can make a conscious effort to keep your eyes moving to search intersections and check your rearview mirror more often. But when your eyes begin to resist those movements, says Mottola, you must get off the road. Find a safe place such as the well-lighted parking lot of a fast-food restaurant, park the car, and lock the doors. Close your eyes and relax for 15 minutes. If you fall asleep, you know it was needed. But if this pattern occurs often, investigate the possibility that you may have a sleep disorder.

Nearly 6.3 million crashes occurred in the U.S. in 1999.

About 2 million people were injured and 37,043 died.

Midnight to 3 A.M. on Saturdays and Sundays were the deadliest 3-hour periods.

38% of fatal crashes involved alcohol.

The highest number of crashes and fatalities occur among teenagers (age 16-19); next come drivers in their early 20s (ages 20-24) and elderly drivers (age 70 and over).

More men than women die each year in car crashes. The reason: Men typically drive more miles and more often engage in risky behave like not wearing a seatbelt.

The good news: Cars are getting safer. About half of 1999-2001 models tested in a crash test earned good ratings, as compared with one in for the 1995-1998 models.

SOURCES: THE INSURANCE INSTITUTE FOR HIGHWAY SAFETY; THE NATIONAL TRAFFIC SAFETY ADMINISTRATION