The risk of 'hangover' effects, e.g. residual daytime sleepiness and impairment of psychomotor and cognitive functioning the day after bedtime administration, is one of the main problems associated with the use of hypnotics. However, the severity and duration of these effects varies considerably between hypnotics and is strongly dependent on the dose administered. This article reviews epidemiological evidence on the effect of hypnotics on patients' risk for accidents such as traffic accidents, falls and hip fractures (i.e. end-points for residual effects). Information on the duration and severity of residual effects of 11 hypnotics (flunitrazepam, flurazepam, loprazolam, lormetazepam, midazolam, nitrazepam, temazepam, triazolam, zaleplon, zolpi-dem and zopiclone) was derived from expert ratings, a meta-analysis and actual driving studies. Epidemiological studies show that the risks of an accident increase with increasing half-life of the hypnotic, but that the use of hypnotics with a short half-life, such as triazolam, zopiclone and zolpidem, can also be associated with increased risks. A summary of results from experimental studies should enable prescribing clinicians to compare residual effects of the various hypnotics at different doses and select the one considered most favourable in this respect for the individual patient. This information should also enable them to inform patients more adequately about the likelihood and duration of residual effects of a specific hypnotic dose.