Fatigue and driving: Disentangling the relative effects of time of day and sleep deprivation

Fatigue is a recognised problem in the transport industry. It has been attributed to two main influences: time of day and the time since last sleep. While the role of these two influences in producing fatigue is not disputed, there is less agreement on how they interact with one another. The aim of this study was to examine the relative effects of time of day and sleep deprivation on fatigue and performance. Two independent groups were exposed to 28 hours of sleep deprivation. For one group (n=39) the sleep deprivation condition began at 06:00 hours and for the second group (n=22), the sleep deprivation condition began at 00:00 hours. By varying the start time for each of the two groups, but keeping constant the duration of sleep deprivation it was possible to examine the effects on performance of variations in the time of day of testing. For the group commencing at 0600 hours the longest period without sleep occurred close to the low point of the circadian rhythm. For the group commencing at 0000 hours, the circadian low point coincided with only around two to six hours of sleep deprivation. Eight computer-based performance tests were used as well as subjective ratings of fatigue. The results showed a clear interaction effect. Both time of day and sleep deprivation factors affected performance but only in combination, neither had independent effects. If the circadian rhythm was not at its low point or trough, there were no effects of sleep deprivation. Performance at the circadian low point was not adversely affected when the study participant was rested. These findings have clear implications for good fatigue management, indicating that night work including driving can be performed if the person is properly rested, but not if night driving is required after long periods without sleep.