Re-entrainment of circadian rhythm of plasma melatonin on an 8-h eastward flight

To estimate the process of re-entrainment we measured the melatonin rhythm on an eastward flight. After the baseline study, 24-hour blood sampling of six male subjects was done on the first and fifth days. During the daytime the subjects were exposed to natural zeitgeber outdoors every day except th e blood sampling day. They were analyzed with an illuminometer when under t he bright light condition. Four of the six subjects showed orthodromic re-e ntrainment, another subject showed antidromic re-entrainment, and the other subject kept the baseline pattern of plasma melatonin. The rate of re-entr ainment in orthodromic re-entrainment was about 55 in per day. Measuring th e circadian rhythm of plasma melatonin has clarified the interindividual re -entrainment difference.