The study participants drove 125 miles on the same highway in France between 6:00 p.m. and 7:30 p.m. and then 3 times between 2:00 a.m. and 3:30 a.m. In preparation for the early morning drives, patients drank a half cup of either caffeinated coffee or decaffeinated coffee or took a 30-minute nap in the parked car at 1:00 a.m. The cars were equipped with a special video camera that recorded when the driver inappropriately crossed the center line during each drive. A professional driving instructor accompanied each participant and also monitored line crossings, as well as driving speed. The cars had another set of controls so that the instructor could take over if needed. After the experiment, the researchers compared the number of line crossings in each of the 4 driving conditions. Studies show that inappropriate line crossings are related to traffic accidents.