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Usually, the symptoms of jet lag are relatively mild, limited to daytime sleepiness, listlessness, headaches, disorientation, mild confusion and generally feeling “out of sorts”. But it can manifest in more classic sleep deprivation symptoms like fatigue, poor performance and alertness, memory lapses, indigestion and gastrointestinal problems, mood swings, etc. Jet lag disorder is sometimes recognized as a full-blown sleep disorder (despite the fact that it is usually self-inflicted), although is almost always quite short-term, generally resolving itself within a week or so. Interestingly, studies of international marathon runners have shown that habitual or repetitive motor tasks not requiring much deep thought (e.g. running), are relatively unaffected by jet lag or poor sleep, while tasks requiring the anaysis of complex problems are much more affected.
Individual circadian periods vary, ranging between 23.5 and 24.5 hours in humans, with a mean of around 24.2 hours (see the section on Circadian Rhythms). How quickly any one individual is able to adjust to a new time zone depends on the period of their particular circadian clock (i.e. how much more or less than 24 hours their particular circadian period is) and the direction of travel. About three-quarters of people have a circadian clock with a period of slightly more than 24 hours, and so typically find it easier and quicker to adjust to westward travel (which requires a delay in the circadian clock to adapt, as daily schedules of meals, sleep, bowel habits, etc, are all pushed back) than eastward travel (which requires an advance in the circadian clock, as daily schedules are pushed ahead). The other quarter of people, who have circadian periods of less than 24 hours, find the opposite. Most people, then, find westward travel easier, and this has has been graphically demonstrated by a study of professional baseball teams, which tend to win significantly more often when travelling westward than eastward (although they win even more often when playing at home or not travelling out of their own time zone at all).
Depending on how many times zones are passed through, it usually takes a few days to naturally adjust to a new time zone, and for the body to gradually reset its sleep-wake cycle (an hour a day is a rough rule of thumb). Interestingly, though, the secondary biological clocks in our body, such as in the liver, heart, pancreas, kidneys, lungs, etc (see the section on Circadian Rhythms for more details), tend to take substantially longer to adjust their daily oscillations and rhythms, and the liver in particular can take several weeks to adjust.
While actually on the plane, the effects of jet lag can be mitigated a little by drinking plenty of water, avoiding alcohol and caffeine, and getting some exercise by walking around, stretching and moving muscles, although it is difficult to prevent or avoid the effects completely. Some studies suggest that fasting before and during a flight (for at least 12 hours), followed by normal meals at local times, can help to quickly recalibrate the body clock, using meal-time zeitgebers to override the usual light-dark cues.
Seasoned frequent flyers and professional flight attendants who fly on a regular basis still suffer from jet lag, and there appears to be no way develop an "immunity". A few other simple things that can also be done on arrival to help expedite the adaptation process, include early extended exposure to outdoor sunshine, and taking over-the-counter melatonin before local sleep time, but unfortunately there is no silver bullet.
Bright light therapy (timed light and dark exposure) is recommended by some scientists as a treatment for jet lag, and a whole science is beginning to grow up around this premise. For example, bright light exposure during normal nighttime hours will tend to delay the circadian clock, while light during the daytime helps to advance it, and this can also be fine-tuned by applying the light exposure at very specific times during the changing circadian cycle, according to the individual’s phase response curve (which measures the changing effects and responses of perturbations to their circadian system).