Aim:
This study aimed to investigate changes in carbohydrate metabolism in the liver of rats under light and dark deprivation and evaluate the effects of melatonin treatment.

Material and methods:
Photoperiodic changes were simulated over two weeks: the natural equinox (March 16-29); the artificial equinox (12:12 light-dark cycle, 500 lux); constant light (24 hours, 500 lux) for dark deprivation; and constant dark (24 hours, 0-0.5 lux) for light deprivation. Forty-eight rats were divided into control and melatonin-treated groups (5 mg/kg daily for 14 days). Enzyme activities and glycogen content in the liver were measured using standard methods. Statistical analysis was performed using the Student’s t-test.

Results:
Glucose-6-phosphate dehydrogenase activity decreased by 18% under constant light but increased by 35% under constant dark compared to the equinox. Pyruvate kinase activity decreased by 17%, while glucose-6-phosphatase and lactate dehydrogenase activities increased by 9% and 20%, respectively, under constant light. Constant dark and melatonin treatment reduced glucose levels by 26% across all conditions, activated aerobic pathways, and increased glycogen content by 13% compared to the equinox.

Conclusions:
Melatonin treatment improved carbohydrate metabolism in the liver of rats under light and dark deprivation, suggesting its role in metabolic adaptation to altered photoperiods.