There are two overriding concerns in the design of schedules for aircrew who have to cope with irregularity of their sleep due either to work at unusual times of the day or to time zone changes. The first is that aircrew must be able to achieve an acceptable sleep pattern, and this is likely to involve short periods of sleep and naps. These, in turn, depend on limits to duty hours. The second is that the arrangement of work must bear in mind that prolonged duty and duty at certain times of the day may both involve relatively low levels of performance, and modelling can identify the adverse interactions of these influences. It is also important that the quality of the sleep of the individual pilot is taken into consideration when there are undue difficulties in coping with irregularity of work.