Long distance flights with rapid time zone shifts of more than 3 hours lead to a dissociation of the inner circadian clock to the outer pacer. Additionally, the different endogenous circadian rhythms will not longer be synchronized by the endogenous pacer melatonin. This leads to complaints like sleepiness, sleep-disturbances and others. These symptoms are called jet-lag. The subject's performance is disturbed, as well. Different studies showed smaller problems with jet lag when travelling to the west. Since the inner circadian rhythm tends to be 24 up to 26 hours, travelling to the west with a prolongation of the daylight will be tolerated better than flights to the east with a shortening of the day length. Rapid time zone shifts with more than 8 hours to the east lead to individual different ways of resynchronization. Subject either try to adapt to the new time zone by shortening the day (backward adaptation) or they resynchronize forward with a longer duration of adaptation time. Elder subjects with already diminished circadian hormonal rhythm often get more problems concerning symptoms of the jet-lag and in time to recover from the disturbance of the inner clock. No differences can be found between business travelers, tourists and high-performance sportsmen. After flights to the west within 3 to 7 days, most of our inner circadian rhythm will be re-synchronized. After flights to the east, resynchronization can take 5 to 14 days. A symptomatic therapy of jet lag symptoms with a short-acting benzodiazepine like triazolam in a dosage of 12.5 mg or less is well tolerated. A therapy with oral melatonin in a dosage of 0.5 to 5 mg/day given in the evening 1 to 2 hours prior to the desired sleeping-time may be helpful for a group of subjects. Another group of subjects will not have any benefit of a therapy with melatonin, but cannot be defined in advance. A recommendation for a therapy with melatonin to treat jet lag symptoms cannot be given at the moment, since scientific data are still missing. Additionally, the same contaminants, which caused some deaths, as in the related substance tryptophan has been found in some tablets of melatonin.