Drowsy driving increases risk of traffic accidents. A major problem remains in the identification of drowsy drivers at risk for traffic accidents. Drowsy driving is the consequence of various behavioural factors (e.g. sleep duration, work duration, shift-work schedules) combined or not with sleep and iatrogenic disorders (e.g. obstructive sleep apnoea syndrome, hypersomnia, drug-induced sleepiness). Severity of sleep disorders is a non-linear predictor of traffic accident risk. In comparison, sleepiness at the wheel (SAW) can be considered as a reliable indicator of a combination of behavioural and sleep disorder factors, and is a better risk predictor. It remains thus very important to question patients about SAW when clinicians have to determine the medical fitness to drive of such patients. Because of the potential risk of under-reporting of SAW, especially in professional drivers, objective measures can help to complement the clinical evaluation. Further researches are needed to optimize objective measures able to predict the risk of traffic accidents due to drowsy driving.