Drivers' sleepiness and falling asleep while driving account for a considerable proportion of vehicle accidents (studies show different results from 1% to 30%). Sleepiness is rarely well recognised as a causing factor of traffic accidents. 2.5% up to 20% people suffer from excessive daytime sleepiness (EDS) with sleep deprivation as its most frequent cause. There is a strong association between sleep deprivation and medical problems--especially sleep disturbances. The sleep apnoea syndrome (SAS) has been identified as the most common cause of habitual drowsy driving. Patients with SAS (apart from other health problems) are 6 times more likely to have accidents. After adequate treatment of severe SAS with continuous positive airway pressure the risk of accident lowered 5 x. Other important sleep disturbances include chronic insomnia, narcolepsy, restless legs syndrome and periodic limb movement in sleep. Sleepiness was described in Parkinson's disease, dementia, epilepsy, in chronic cardiacs and in people with complex internal health problems. Regular or single intake of drugs (benzodiazepines, antidepressants, antihistaminics, antipsychotics and others) can itself induce sleep problems. Sleepiness in persons without sleep disorder may occur due to preventable causes such as poor sleep habits which lead to sleep deprivation.