Melatonin synthesis:: A possible indicator of intolerance to shiftwork

Hypothesis Melatonin synthesis, which is directly controlled by the central circadian pacemaker indicates the circadian phase better than rectal temperature. Methods Thirty four men (16-32 years, 7 morning, 13 neither, 14 evening types) performed a constant routine (24-26-hr bedrest, <30 lux, 18-20degreesC, hourly isocaloric diet). Salivary melatonin level was determined hourly and rectal temperature was continuously recorded. Results The nadir of rectal temperature occurred 1.5 hr (P = 0.017), the onset of melatonin synthesis 3 hr earlier (P < 0.0001) in morning than in evening types. Morningness was not related to the quantitative but significantly to the temporal parameters, closer to those of melatonin than of rectal temperature. Conclusions The melatonin onset is a more reliable indicator of the diurnal type than the nadir of rectal temperature. As morningness has been associated with intolerance to shiftwork, melatonin profiling provides a suitable basis for the establishment of directed preventive measures. (C) 2002 Wiley-Liss, Inc.