Every day, thousands of travellers fly across many time zones arriving in new environments which are at a very different point in the daily routines of work and rest, sleep and wakefulness, and day and night. This rapid shift of temporal referents requires a change of psychological and physiological patterns so that the individual can accommodate to the new time schedule. This process of accomodation is frequently accompanied by malaises, including gastric distress, fitful sleep, irritability, fatigue, apathy, and mild depression. The occurrence of these symptoms highlights the close relationship that normally exists between an individual's psychological and physiological rhythms and environmental schedules.