In general, adjustment to a new time zone takes one day for each hour of time difference. Almost all of the body’s physiological processes have a rhythm, or pattern, that varies over the course of a day. The most obvious of these circadian rhythms are sleep and wakefulness, which are driven by physiological responses to light and dark; the internal body clock also controls alertness, hunger, digestion, urine production, body temperature, and the secretion of hormones. When these rhythms have been disrupted, they cannot all be brought back into synchrony at the same rates once the destination has been reached.

A stress hormone that is secreted in a circadian pattern and that is particularly sensitive to interruptions in sleep-wake cycles is cortisol. Levels of cortisol, which normally increase during the day and decrease at night, are found in unusually high levels in people who experience jet lag on a regular basis (e.g., flight attendants and pilots). Brain scans and memory performance tests of such crew members, who often work multiple transmeridian flights with brief in-between flight “recovery” times, show that they have reduced temporal lobes and poor short-term memory. Increasing cortisol levels corresponded with decreasing temporal lobe size in these individuals, suggesting a direct link between physiological desynchronization and decreased functionality of short-term memory. Fortunately, once synchronization is reestablished, short-term memory returns to its normal state.