Each year millions of travellers experience a temporary loss of sleep, discomfort, and reduced efficiency after rapid travel across several time zones-the experience commonly known as" jet lag." In no way is jet lag associated with serious illness in normal people, but after transmeridional flight major body functions such as sleepiness, hunger, and defecation may intrude at inappropriate times for variable periods. Disturb-ances of biological rhythms are generally accepted as largely responsible for the observed effects. Since, however, deprivation of sleep is a major complaint among jet-lagged travellers loss of sleep by itself may be a factor in impaired performance. Biological rhythms are not to be confused with the discredited so-called biorhythms of a 23-day physical cycle, a 28-day emotional cycle, and a 33-day intellectual cycle,'which have given rise to calculators to pinpoint propitious or un-propitious days according to the individual's date of birth. The existence of biological rhythms is verifiable by experiments and they are found in all species from unicellular organisms to man, and with frequencies varying from fractions of a second (for example, the firing of nerve fibres) to years (for example, variations in population). Major biological rhythms coincide with geophysical events in the environment: the daily light-dark cycle, the lunar and tidal cycles, and the seasonal cycle. Many rhythms have been shown to continue even in the absence of environmental cues or" zeitgebers"(time givers), but with periods slightly different from the environmental period-hence the term circadian (circa, about; dies, a day) for daily rhythms which continue, or free run, in isolation. In view of this endogenous nature of many rhythms, organisms are thought to have developed an internal model of external time, usually referred to colloquially as the biological clock. The major dailycycles in man (that is, those relating to rest-activity (sleep-wake), temperature, rapid eye movement sleep, excretion of electrolytes, and production of cortisol) have been