Melatonin synthesis: a possible indicator of intolerance to shiftwork

Area of Science:

• Chronobiology
• Sleep Medicine
• Physiology

Background:

• Circadian rhythms govern physiological processes, influencing diurnal types (morningness/eveningness).
• Accurate assessment of diurnal types is crucial for understanding individual differences in sleep-wake cycles and their implications.
• Melatonin and rectal temperature are commonly used biomarkers for circadian phase assessment.

Purpose of the Study:

• To compare the reliability of melatonin synthesis onset and rectal temperature nadir as indicators of diurnal type.
• To investigate the relationship between morningness/eveningness and temporal parameters of melatonin and temperature rhythms.

Main Methods:

• A constant routine protocol was applied to 34 men (16-32 years) categorized as morning, neither, or evening types.
• Hourly salivary melatonin levels and continuous rectal temperature were recorded over 24-26 hours.
• Statistical analyses were performed to compare temporal parameters between diurnal types.

Main Results:

• The onset of melatonin synthesis occurred significantly earlier in morning types compared to evening types (P < 0.0001).
• The nadir of rectal temperature also occurred earlier in morning types, but to a lesser extent (P = 0.017).
• Morningness was more strongly associated with the temporal parameters of melatonin than rectal temperature.

Conclusions:

• The onset of melatonin synthesis is a more precise and reliable marker for diurnal type classification than the nadir of rectal temperature.
• Melatonin profiling offers a robust method for identifying diurnal types, which is relevant for managing shiftwork intolerance.
• These findings support the development of targeted preventive strategies based on individual chronotypes.