A researcher tested the effect of caffeine on sleep. Forty pilots volunteered to participate in the study and signed up for one of two nights. Pilots on Tuesday night drank a cup of regular coffee, and those on Wednesday night received a cup of decaffeinated coffee. They then took a nap, and the length of time to fall asleep was recorded with the aid of an EEG. Pilots in the caffeine group took significantly longer to fall asleep. The researcher concludes that caffeine interferes with the ability to fall asleep.