Exposure to Dim Light at Night Impairs Photoperiodic and Metabolic Gene Expression in Syrian Hamsters ( Mesocricetus auratus )

Rapid urbanisation has significantly increased nighttime illumination, thereby affecting the photoperiod perception of seasonal‐breeding animals. In this study, we investigated the effect of dim light (0.0064 W/m 2 ) combined with an extended photoperiod (15 L:9D/day) on the hypothalamic‐pituitary‐gonadal (HPG) axis in adult (~ 4 months old) Syrian hamsters ( Mesocricetus auratus ). Two groups of male animals were used: group one (LD; n = 5) was exposed to long photoperiod (15 L:9D), while group two (dLD; n = 5) experienced the same photoperiod with added dim nocturnal light (0.0064 W/m 2 ) for 4 weeks. No significant change in body mass, testicular volume, plasma testosterone, cholesterol and triglycerides was observed at the end of the study. However, analysis of hypothalamic tissue revealed that nighttime light exposure led to upregulation of the Eya3 transcript and downregulation of genes involved in photo responsiveness and reproduction ( Tshβ, Dio2, GnRh, Six1 ), with concurrent downregulation of epigenetic markers ( Tet2, Hat1 ), and increases in Dnmt1, Dnmt3A , and Hdac3 . Additionally, there was a downregulation of steroidogenic transcripts ( StAr, Cyp17a1, Esrrb, Ar, Prmt5, Nsdhl ) in the testes of dLD animals, while the Hdac3 transcript was upregulated in testicular epigenetic transcripts. The hepatic metabolic and epigenetic gene expression also differed significantly. Genes such as Acc , Fasn , Hmgcr , Srebf1 , Srebf2 , Tet1 , and Hat1 were significantly lower under dim‐light exposure, while Dnmt3a , Hdac3 , and Hdac5 were upregulated. These findings suggest that dim night light can disrupt the photoperiodic sensing pathway of seasonal reproduction, highlighting the ecological effects of artificial light on wildlife in urban settings.