Re-entrainment of the circadian rhythms of plasma melatonin in an 11-h eastward bound flight.

We investigated the re-entrainment of melatonin rhythm in an 11-h eastward-bound flight. Eight male subjects participated in the present study. Blood sampling was carried out once before the flight and twice after the flight. During the daytime the subjects were exposed to natural zeitgeber outdoors on the day except the blood sampling. Seven of eight subjects showed antidromic re-entrainment, and the other subject showed orthodromic re-entrainment. The intensity of natural day light in New York amounted to 20 000 lx. As for the direction of the re-entrainment in New York the antidromic re-entrainment is naturally dominant.