Effects of “Jet Lag ” on Hormonal Patterns. II. Adaptation of Melatonin Circadian Periodicity*

Abstract

The plasma melatonin concentration was measured by RIA in samples obtained at 15-min intervals during seven 24-h studies staggered over a period of 10 weeks (October to January). The five normal male volunteers underwent a 7-h westward time shift by jet preceding the second study and, 1 month later, a 7-h eastward shift preceding the fifth study. In the basal, unperturbed state, the 24-h plasma melatonin profile was characterized by a nocturnal elevation, approximately 2.5-fold above the mean daytime value, occurring at 0130 ± 0039 local time, thus 2.5 h after exposure to darkness and 1.5 h after sleep onset. Episodic fluctuations occurred throughout the entire 24-h period. A significant decrease in the 24-h sleep and daytime mean melatonin levels was observed only after the westward trip, during which the difference between sunrise and sunset (daylight duration) was 16 h rather than 10 h. Shifts in the acrophase observed on the day after the westward trip indicated partial adaptation (>2-and <5-h shift), while the eastward trip, which involved 33 h of sleep deprivation, caused a total desynchronization. Eastward displacement was also associated with a significant disruption in the sleep pattern and an increase in the anxiety and depression scores. On the 11th day after trips in both directions, the melatonin acrophases were fully adapted to the local time. Despite an apparent correlation between the melatonin and cortisol rhythms during the unperturbed state, a lack of relationship is supported on the basis of the following discrepancies: diminished 24-h mean level of melatonin after westward displacement without changes in the 24-h mean levels of ACTH and cortisol, more rapid adaptation of the circadian rhythm of melatonin without the eastward-westward time differences observed for cortisol, and abolishment of the 6-h time difference between melatonin and cortisol rhythms during the period of adaptation after time shift. The amplitude of the acrophase declined throughout the period of investigation in parallel with the decrease in daylight duration. This change, apparently unrelated to acute time shifts or duration of exposure to artificial light, suggests that daylight may exert a prevailing influence on the amplitude of the melatonin rhythm.