Toshiharu Takahashi MDDepartment of Psychiatry, Aoto Hospital, Jikei University School of Medicine, 6-41-2, Aoto, Katsushika-ku, Tokyo 125–8506, Japan.
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 the blood sampling day. They were analyzed with an illuminometer when under the bright light condition. Four of the six subjects showed orthodromic re-entrainment, another subject showed antidromic re-entrainment, and the other subject kept the baseline pattern of plasma melatonin. The rate of re-entrainment in orthodromic re-entrainment was about 55 min per day. Measuring the circadian rhythm of plasma melatonin has clarified the inter-individual re-entrainment difference.
We investigated the sleep–wake rhythm, sleep architecture and rectal temperature of jet lag syndrome.1–2 We could not estimate exactly the process of re-entrainment, because such parameters had a masking effect. If the light condition is completely controlled, the plasma melatonin rhythm is considered to be important as it is the most reliable overt rhythm of the circadian oscillator. Here, we could measure the melatonin rhythm on an eastward flight, from Tokyo (TYO) to Los Angeles (LAX), which had an 8-h time zone difference.
SUBJECTS AND METHODS
Six male subjects aged 29–45 participated in this study. All subjects were healthy physically and psychologically. We carried out 24-h blood sampling to assess their plasma melatonin rhythm, and examined the process of re-entrainment of the circadian rhythm.
In TYO, the sleep time was scheduled from 00.00 to 08.00 h. Baseline blood sampling was done every 90 min on 22 September 1997. We started for LAX on 27 September. In LAX blood sampling was done on the first day and on the fifth day at local time. The subjects ate their meals together. Turning on lights was forbidden except footlights with about 10 lux from 00.00 to 08.00 h. During awakening time on the day when blood sampling was done, the windows were blacked out and the light intensity was made to be 500 lux or under. The subjects were allowed to associate with one another to avoid being alone in order not to take a nap. In LAX from the second day to the fourth day, the subjects were exposed to the natural zeitgeber outdoors during their time awake. We monitored the intensity of light the subjects were exposed to with the portable illuminometer. Blood samples were immediately centrifuged and serum was stored in the refrigerator. After they were taken back to Japan with dry ice, the serum level of melatonin was assessed with radioimmunoassay. On the phase shift of melatonin rhythm, we analyzed the rising phase-point and the falling phase-point, which were the mid-points of the peak.
1. Four of the six subjects showed orthodromic re-entrainment, another subject showed antidromic re-entrainment, and the other subject kept the baseline pattern of plasma melatonin in LAX (Fig. 1).
2. The rate of re-entrainment in this field study was about 55 min per day in orthodromic re-entrainment. The amplitude of the circadian melatonin rhythm was not dumped on the first day in LAX.
3. The intensity of the natural daylight the subjects were exposed to in LAX was more than 3000 lux. In other words all of them were under the bright light condition. There were no differences between the orthodromic and antidromic re-entrainment with regard to the timing of the exposure to and the intensity of light (Fig. 2).
The antidromic re-entrainment has rarely been reported in field studies and the mechanism has not been elucidated.3 The past study of jet lag syndrome has the inevitable problem with inter-individual differences.4–5 Measuring the circadian rhythm of plasma melatonin has clarified the inter-individual difference of re-entrainment. The rate of re-entrainment by measuring the plasma melatonin has corresponded to the rate reported by Klein et al. on an eastward flight.3 The subjects were exposed to natural daylight from 08.00 to sunset. This time included subjective morning at home time. We recognized the light intensity of natural daylight in LAX which was more than 3000 lux, which reached over 12000 lux at the maximum level. Sunlight in LAX can be useful to entrainment on an eastward flight. With regard to the timing of exposure and the intensity of light we could not find any difference between the orthodromic and the antidromic entrainment. Further study is necessary to investigate the mechanism of antidromic re-entrainment and how to prevent it.