Jet lag disorder refers to sleep–wake disturbances associated with rapid travel across time zones, resulting from a transient mismatch between the external physical/social environment and the timing of the individual's endogenous circadian rhythm. Symptoms include difficulty falling asleep at night, daytime sleepiness, general malaise, gastrointestinal upset, and mood changes (Winget et al. 1984). Symptoms of jet lag are transient, with resolution as the traveler adjusts physiologically and socially to the destination time zone. The severity and duration of symptoms are related to the number of time zones crossed. Eastward travel, in which adjustment requires an advance shift of circadian rhythms, is usually more difficult than westward travel and affects up to 80% of business travelers (Wagner 1996). Sleep-onset difficulties are more common with eastward travel, and evening sleepiness and sleep maintenance problems are more prominent with westward travel (Boulos et al. 1995). Factors such as anxiety, air quality, lack of physical activity, and dehydration also contribute to the sleep loss and malaise that often accompany jet travel.