The role of driver sleepiness in car crashes: A review of the epidemiological evidence

Abstract

Everyday experience, knowledge about the physiology and sleep, and research under controlled conditions all suggest that driving ability is impaired by sleepiness. Epidemiological studies attempt to characterize and quantify the association between sleepiness and the risk of crashing under usual driving conditions in real populations, with the overall goal of preventing injuries. All studies are hampered to some degree by methodological challenges that arise from the multi-causal and acute nature of car crashes, and the difficulties of defining and measuring sleepiness in this context. Despite the challenges, there is considerable evidence that sleepiness and its determinants contribute to car crashes and related injuries. Sleep deprivation, driving at time of circadian impairment, and self-identified sleepiness have been shown to increase risk. The magnitude of the risk cannot be precisely estimated due to heterogeneity in study designs, in populations studied, and in measurement of sleep-related variables. However it appears that risk increases substantially with less than 5 hours sleep in the previous 24 hours and when driving in the early hours of the morning. Some groups in the population are at particular risk, including young adults, shift workers, people with sleep disorders and people driving long distances. The road environment appears to modify the risk associated with sleepiness, with monotony exacerbating the effect. There also appears to be an important interaction of sleepiness with alcohol, even at very modest levels of drinking. Taken together, the evidence suggests that 15-20% of car crashes may be attributable to driver sleepiness in high income countries, although there is still uncertainty about this. © 2009 Birkhéuser Verlag/Switzerland.