The relationship between the rate of melatonin excretion and sleep consolidation for locomotive engineers in natural sleep settings

Background: The aim of the study was to examine the role that melatonin production plays in the regulation of sleep consolidation in a population of shiftworkers working and sleeping in their natural environments. Methods: 253 locomotive engineers (249 male, 4 female, mean age = 39.7 years) participated in the study for a 2-week period whilst working their normal roster patterns. Participants recorded details for all sleep periods in a sleep diary and collected urine samples during each day's main sleep period. The samples were subsequently assayed for the metabolite of melatonin in urine, 6-sulphatoxymelatonin (aMT6s), and the rate of excretion during main sleep periods was calculated. Results: Separate one-way factorial ANOVAs revealed a significant effect of time of sleep onset on aMT6s excretion rate, sleep duration, and subjective sleep quality. Generally, the rate of aMT6s excretion was lower, sleep duration was shorter, and sleep quality was lower for sleeps initiated during the daytime than for sleeps initiated at night. Conclusion: Combined with previous studies linking melatonin production and sleep propensity, and others demonstrating the relationship between sleep consolidation and melatonin production in forced desynchrony protocols, the current results indicate that low production of melatonin may play a role in the poor consolidation of daytime sleep in natural sleep settings.