Investigating the relative effects of sleep deprivation and time of day on fatigue and performance.

Time of day and the time since last sleep are acknowledged causes of fatigue, but comparatively little is known about how they interact. This study examines the relative effects of time of day and sleep deprivation on fatigue and performance. Two independent groups were exposed to 28 h of sleep deprivation beginning at 06:00 h for one group (n = 39) and at 00:00 h for the other (n = 22). By varying the start time for the two groups, but keeping constant the duration of sleep deprivation, the effects of variations in the time of day of testing could be examined. For the 06:00 h start group the longest period without sleep occurred close to the low point of the circadian rhythm. For the 00:00 h start group the circadian low point coincided with only two to six hours of sleep deprivation. Performance was evaluated two-hourly using eight computer-based tests and subjective fatigue ratings. The results showed a clear interaction effect. Both time of day and sleep deprivation affected performance but only in combination; neither had independent effects. These findings have implications for fatigue management.