Urbanization and human activities are increasing global levels of artificial light at night (ALAN). Several studies have shown that ALAN negatively impacts animals, altering their perception, physiology, and behaviour, eventually leading to reduced fitness. ALAN also disrupts biological rhythms, affecting animals' ability to synchronize with natural cycles. There still is a critical lack of knowledge on ALAN's effects on freshwater ecosystems, which are highly threatened biodiversity hotspots. Our study aimed to understand the molecular effect of prolonged ALAN exposure in the zebrafish, a model species in this field. Using high-throughput RNA sequencing, we investigated the transcriptome-wide gene expression responses in whole brains of ALAN-exposed zebrafish. Samples were collected at four different timepoints along the day, two at daytime and two at nighttime, to identify daily changes in gene expression and metabolic pathways. A one month exposure to ALAN varied the expression of several genes and metabolic pathways, both at night, when zebrafish were directly exposed to artificial light, and during the day, at natural light conditions. The highest number of differentially expressed genes were found at the early hours of the day and at night. Overall, circadian clock genes and those related to immunity, stress response, responses to sensory stimuli, energy production, motor activity, and reproductive processes changed in expression levels. These results provide new insights into the mechanisms through which ALAN affects animal biology, suggesting broader impacts than previously recognised. To safeguard aquatic ecosystems and their biodiversity, it is paramount to mitigate light pollution.
