Nearly everyone who has ever taken a long-distance car trip has suddenly found their eyelids drooping and their concentration waning as the night grows later. But if you don’t want to stop, what’s the best way to counteract increasing drowsiness, coffee or a quick nap? Researchers in France have determined the answer rests largely in your age.

The scientists set out to test the performance of two-dozen sleep-deprived motorists by first making them drive a two-mile course between six and seven-thirty p.m., and then again between two a.m. and three-thirty p.m. One group of drivers was given decaffeinated coffee, another received regular coffee and a third was allowed to take a thirty-minute nap.

The decaf drinkers performed the worst, crossing the highway’s lines one-hundred-fifty-nine times. Nappers did better, with eighty-four line crossings. But the coffee drinkers had the best performance, crossing lines just twenty-seven times.

The difference was even more startling among drivers between the ages of forty and fifty. Coffee reduced their chance of swerving by eighty-nine percent, while a nap only reduced it by twenty-three percent. But among the younger drivers, the nap was nearly as effective as the caffeine.

According to the researchers, the younger drivers benefited more from the nap because they were able to sleep longer and more deeply than the older test subjects.

Still, while a cup of coffee may help a drowsy driver, the safest option is always to pull over, stop for the night and get a good night’s sleep.