Transient insomnia is known to occur in people who normally sleep well but whose sleeping pattern has been altered (e.g. by shift-work, intercontinental travel). The occasional use of an hypnotic is likely to be beneficial in these cases although, in the past, the problem of daytime sedation precluded the use of such drugs in certain occupations. Recent advances in therapeutics reflect the increasing interest in the impairment of performance and it is now recognized that both the nature and persistence of any effect must be determined. Two broad approaches are used at present: the laboratory study of isolated skills and some form of simulation such as car handling. There is, however, clear disagreement concerning the relevance of these two methods to the real-life situation and both have their limitations, so results must be interpreted with caution. In addition, an inability to demonstrate impaired performance does not necessarily mean that the drug is completely free from adverse effects, and different effects may be seen in a patient population compared with healthy volunteers because of variations in age, gender or concomitant drug therapy etc. It is fortunate that there are many drugs within a specific group which, though they have similar efficacy, have different effects on performance and so, accepting a limited impairment of performance, it should be possible for therapy to be suited to the day-to-day needs of any individual patient.