Melatonin synthesis occurs earlier in the morning than in the evening types who strictly adhere to their individual time schedule. This study tested whether melatonin profiles still separate between diurnal types who vary their individual rhythm or who are submitted to prescribed time schedules. Male and female students were observed during a constant routine in the laboratory (24-26 hours bed rest, 20 degrees C, <30 lux, hourly isocaloric diet) and soldiers who spent several days in a military hospital were observed during 12 hours (bed rest, <30 lux, normal meals). Salivary melatonin levels were determined hourly. In both studies, melatonin profiles occurred earlier in the morning than in the evening types. The difference was smaller in soldiers, thus conditioning contributes to the actual phase position but does not mask morningness. As morningness is related to the ability to cope with shift work, the melatonin onset can be used as a criterion when assigning a person to shift work.
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