Background: Sleep-related accidents often involve healthy young persons who are driving at night. Coffee and napping restore alertness, but no study has compared their effects on real nighttime driving performances.
Objective: To test the effects of 125 mL of coffee (half a cup) containing 200 mg of caffeine, placebo (decaffeinated coffee containing 15 mg of caffeine), or a 30-minute nap (at 1:00 a.m.) in a car on nighttime driving performance.
Design: Double-blind, randomized, crossover study.
Setting: Sleep laboratory and open highway.
Participants: 12 young men (mean age, 21.3 years [SD, 1.8]).
Measurements: Self-rated fatigue and sleepiness, inappropriate line crossings from video recordings during highway driving, and polysomnographic recordings during the nap and subsequent sleep.
Intervention: Participants drove 200 km (125 miles) between 6:00 p.m. and 7:30 p.m. (daytime reference condition) or between 2:00 a.m. and 3:30 a.m. (coffee, decaffeinated coffee, or nap condition). After intervention, participants returned to the laboratory to sleep.
Results: Nighttime driving performance was similar to daytime performance (0 to 1 line crossing) for 75% of participants after coffee (0 or 1 line crossing), for 66% after the nap (P = 0.66 vs. coffee), and for only 13% after placebo (P = 0.041 vs. nap; P = 0.014 vs. coffee). The incidence rate ratios for having a line crossing after placebo were 3.7 (95% CI, 1.2 to 11.0; P = 0.001) compared with coffee and 2.9 (CI, 1.7 to 5.1; P = 0.021) compared with nap. A statistically significant interindividual variability was observed in response to sleep deprivation and countermeasures. Sleep latencies and efficiency during sleep after nighttime driving were similar in the 3 conditions.
Limitations: Only 1 dose of coffee and 1 nap duration were tested. Effects may differ in other patient or age groups.
Conclusions: Drinking coffee or napping at night statistically significantly reduces driving impairment without altering subsequent sleep.