Effect of Nighttime Fasting vs. Nighttime Eating on Urinary Melatonin Excretion in Fixed Night Workers: A Randomized Crossover Trial

Introduction: Shift work disrupts circadian rhythms and has been associated with metabolic and sleep disturbances. Melatonin, a key hormone in circadian regulation, is influenced by food intake timing. Emerging evidence suggests that in night shift workers, nighttime food intake may act as a secondary zeitgeber, modulating melatonin secretion and potentially reducing the effects of circadian misalignment induced by shift work. This study aimed to evaluate the effect of nighttime fasting versus nighttime eating on urinary melatonin excretion in fixed night workers, contributing to a better understanding of chrononutrition strategies that may mitigate circadian misalignment and improve long-term health outcomes.

Methods: A randomized controlled, three-condition, crossover trial was conducted with ten metabolically healthy police officers working fixed night shifts. Participants underwent three conditions in randomized order: (1) Night Shift Fasting (fasting during night shift), (2) Night Shift Eating (consumption of a standardized meal providing 30% of daily energy intake at ~02:00), and (3) Nighttime Sleep (control condition). Each condition lasted 48 hours with a minimum six-day washout period. Urinary 6-sulfatoxymelatonin excretion was assessed at five-time intervals. Actigraphy was used to monitor rest-activity cycles and assess potential confounding effects related to variations in physical activity and sleep duration. The controlled dietary protocol ensured that total caloric intake and macronutrient distribution remained constant across conditions, isolating the impact of meal timing on melatonin secretion. Data were analyzed using Generalized Estimating Equations (GEE) models adjusted for age, BMI, activity level, and light exposure. The AUC for 6- sulfatoxymelatonin was calculated using the trapezoid method.

Results: Melatonin excretion was significantly lower in the Night Shift Fasting condition compared with the Night Shift Eating condition (10.61 ± 1.42 versus 14.61 ± 2.85, p = 0.038). Significant differences were observed in melatonin excretion over time within each intervention (p < 0.05). Area under the curve (AUC) analysis showed significantly lower 6-sulfatoxymelatonin excretion in Night Shift Fasting compared with Night Shift Eating (p = 0.020).

Conclusion: Nighttime fasting is associated with reduced urinary melatonin excretion compared with nighttime eating in fixed-shift workers. These findings suggest that nighttime eating may function as a secondary zeitgeber, providing a temporal cue that helps synchronize circadian rhythms and modulate melatonin secretion. Future studies should confirm these results. Support: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

Publication History

Article published online:
08 October 2025