Situational and driver-related factors associated with falling asleep at the wheel

In order to investigate the circumstances under which drivers tend to fall asleep while driving, a questionnaire study was carried out among a sample of 4448 crash-involved car drivers in Norway. The relationship between falling asleep at the wheel and various background factors was investigated by multiple logistic regression analysis. Six per cent of the drivers reported having fallen asleep at the wheel during the previous 12 months, and 22 per cent reported having ever fallen asleep while driving. Male drivers were 2.5 times more likely than females to have fallen asleep while driving. Young drivers are overrepresented in such incidents, probably due to more night-time driving and a lifestyle implying less sleep. Falling asleep at the wheel typically occurs on a straight rural road with little traffic and a relatively high speed limit, and during good weather and driving conditions. Although this may indicate that falling asleep is more likely during monotonous conditions, it is however not possible to make any definite conclusions without knowing how the exposure is distributed across different driving conditions. Drivers fall asleep relatively more often on roads where they seldom drive, and when the trip purpose is vacation, leisure driving, or visits. Between midnight and 6 a.m. the risk of falling asleep at the wheel is about 17 times higher than between 6 a.m. and noon. However, due to a much higher traffic volume during daytime, the number of falling-asleep incidents is higher during the day than during the night. Most drivers who have fallen asleep at the wheel report that they felt tired before the incident, but they tried to defy the symptoms, believing they could keep awake by effort. Very few report that they fell asleep without feeling tired in advance. Those who have felt tired while driving tried various measures to stay awake (opening the window, putting on the fan, playing music, listening to the radio, etc.), but very many report falling asleep despite such measures. A substantial proportion of the drivers fall asleep while driving after poor sleep, or after a particularly long or strenuous day, but most incidents seem to occur without any reported sleep deprivation or fatiguing precursors. On the basis of data regarding annual falling-asleep incidents, proportion of incidents leading to a crash, and annual driving distance, the risk of a sleep-related crash is estimated to be 0.15 crashes per million km. Furthermore, drivers who have fallen asleep at the wheel are more likely to have been the guilty party in their last reported crash, even for crashes not reported to be caused by sleep or fatigue. A possible explanation is that falling asleep at the wheel may be associated with certain more general risk-related personal characteristics.