The circadian melatonin and cortisol secretion pattern in permanent night shift workers

In permanent night shift workers the impact of environmental time cues on the circadian system is conflicting. Rhythm adjustments to the nocturnal work schedule have been described, and their significance for tolerance to shift work is under discussion. Reports concerning the effect of this work situation on the setting of the endogenous clock are, however, inconsistent. We examined nine healthy young male permanent shift workers with high work satisfaction at the end of a week on night work and seven male controls with normal diurnal working hours. All subjects were admitted to a research facility for 28 h, and blood was collected with a continuous-withdrawal pump in portions taken hourly for the estimation of the circadian melatonin (MT) and cortisol secretion pattern. One control did not exhibit a circadian secretion pattern. When compared with the other controls (n = 6), all except one of the shift workers showed no difference in the phase or phase relationship of their hormonal profiles. In the shift workers, a minor trend toward elevation of the MT amplitude and an increase in two indicators for the amount of MT secreted were noticed. One individual displayed inverse hormone rhythms with an undisturbed phase relationship of MT and cortisol and inconspicuous hormone amplitudes. He showed, however, an inverse day-night rhythm in his private life, too. The data collected suggest that even permanent night workers with a high degree of work satisfaction do not usually lose the diurnal orientation of their endogenous clock. Factors other than reorientation of the circadian system may be more important for high tolerance to shift work.