The validity of the temporal parameters of the daily rhythm of melatonin levels as an indicator of morningness.

The present study tested the assumption that the temporal parameters of melatonin synthesis indicate the individual circadian phase position better than the minimum of rectal temperature. Thirty-four men and 17 women (16-32 yr, 14 morning, 14 neither, 23 evening types) completed a constant routine (24-26 h bedrest, <30 lux, 18-20 degrees C, hourly isocaloric diet). Salivary melatonin level was determined hourly, rectal temperature and heart rate were continuously recorded throughout. The minima of rectal temperature and heart rate as well as the temporal parameters of melatonin synthesis occurred earlier in morning than in evening types and were significantly related to the subjective circadian phase as determined by the morningness-eveningness questionnaire. The level of significance and the resolving power was greater for the melatonin parameters and for the minimum of heart rate than for the minimum of rectal temperature. The temporal differences between the opposite diurnal types were larger and the correlation coefficients with morningness were higher in men than in women. The temporal parameters of melatonin synthesis are valid indicators of the individual circadian phase. As morningness is closely related to the ability to cope with shiftwork, the determination of the melatonin profile might be a valuable element of the criteria when assigning a person to shiftwork.