Around a fifth of all road accidents can be attributed to fatigued drivers. Previous studies indicate that driving performance is influenced by time of day and decreases with sustained wakefulness. However, these influences occur naturally in unison, confounding their effects. Typically, when people drive at a poor time of day and with extended wake, their sleep is also restricted. Hence, the aim of the current study was to determine the independent effects of prior wake and time of day on driving performance under conditions of sleep restriction. The driving performance of fourteen male participants (21.8 ± 3.8 years, mean ± SD) was assessed during a 10 min simulated driving task with speed/lane mean, variability and violations (speeding and crashes) measured. Participants were tested at 2.5h intervals after waking, across 7 × 28 h days with a sleep:wake ratio of 1:5. By forced desynchrony each driving session occurred at 9 doses of prior wake and within 6 divisions of the circadian cycle based on core body temperature. A mixed models ANOVA revealed significant main effects of circadian phase, prior wake and sleep debt on lane violations. In addition, three significant two-way interactions (circadian phase × prior wake, prior wake × sleep debt, sleep debt × circadian phase) and one three-way interaction (circadian × prior wake × sleep debt) were identified. The presence of the large interaction effects shows that the influence of each factor is largely dependent on the magnitude of the other factors. For example, the presence of the time of day influence on driving performance is dependent on the length of prior wake or the presence of sleep debt. The findings suggest that people are able to undertake a low-difficulty simulated drive safely, at least for a short period, during their circadian nadir provided that they have had sufficient sleep and have not been awake too long.
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