Dr. Yee wonders whether 1 of our participants could have had a nocturnal visual deficit that could explain the driving handicap recorded during our nighttime driving sessions. All participants were healthy, including their perceptual and psychomotor functions. We used the expertise of our copilots (who are professional driving instructors) to test the normative driving patterns of our participants in a 20-minute session. As Dr. Yee mentioned, the normative data of our participants were homogenous in our paper as in previous studies (1). This would already exclude a diurnal visual problem. Participant 7 behaved just like the other participants during the baseline condition.

A physician interviewed all participants to check for physical conditions that could interfere with driving. In addition, the copilot tested daytime and nighttime visual acuity in a real-world environment (participants were asked to read panels on the side of the road).

We do not believe that a nocturnal visual handicap could result in inappropriate line crossings, such as those of participant 7. If a visual problem had affected driving ability, the participant would not have been able to drive, and this would have resulted in a very large number of line crossings, which was not the case.

Participant 7 reduced the driving handicap by half after the nap. To our knowledge, this countermeasure does not affect vision. Sleepiness, measured by the Karolinska Sleepiness Scale in the middle of the driving session, was also modified by the countermeasures that showed statistically significant differences among the conditions. These results lead us to believe that a nocturnal visual deficit is probably not a valid factor that explains the driving handicap of participant 7.

We looked at the total number of copilot interventions in the driving sessions. Our copilots intervened 4 times during the placebo condition, 2 times during the coffee condition, and 1 time during the napping condition (all participant data pooled). No intervention was needed during the baseline condition. In France, the legal speed limit on freeways is 80 miles per hour, so our participants were driving at a speed similar to that of regular traffic.

Finally, Dr. Yee asked what measures the professional copilot used to stay awake for the study. We used preventive naps in the afternoon and coffee to improve the vigilance of our copilots.