Fatigue, sleep restriction, and performance in automobile drivers: a controlled study in a natural environment.

OBJECTIVES

To test the neurobehavioral consequences of sleep restriction combined with fatigue from long-distance driving (1000 Km/600 miles).

DESIGN

Counterbalanced study involving 3 experimental conditions: laboratory after controlled habitual sleep (8.5 hours), driving after controlled habitual sleep (8.5 hours) (Road 1), and driving after reduced sleep (2 hours) (Road 2).

SETTING

Sleep laboratory and open French highway.

PARTICIPANTS

10 male participants (mean age 22 years, range 18-24 years, mean driving distance per year 15000 Km/9000 miles) free of sleep disorders.

MEASUREMENTS: Simple reaction time, prospective self-assessment of performance, and instantaneous fatigue and sleepiness ratings measured at 2-hour intervals.

RESULTS

A two-way repeated ANOVA with time of day and condition indicated a significant main effect for time of day (p < 0.05). The interaction between the two factors (condition * time of day) was also significant (p < 0.05). The effects of time of day were significant only in the condition of driving after sleep restriction, (p < 0.05). Under sleep restriction, some drivers presented an increase of 650 milliseconds compared to the laboratory condition, representing an increase of 23 meters in breaking distance at a speed of 75 miles per hour. Correlation analyses showed a significant linear correlation between self-assessment and reaction time in the laboratory condition (r = -0.58, p < 0.01) but not in the road conditions. Self-ratings during the breaks showed a significant increase in instantaneous self-rated fatigue and sleepiness between Road 1 and Road 2 conditions (Wilcoxon's test, Z = - 6.47, p < 0.0001 and Z = - 6.26, p < 0.0001).

CONCLUSIONS

Sleep restriction combined with fatigue significantly affects reaction time. The lack of correspondence between reaction time and prospective self-evaluation of performance suggests that self-monitoring in real conditions is poorly reliable.