Background and Aim: Light pollution causes structural and functional changes of many organs, especially those with cyclic activity. Research on the effects of artificial light and light pollution on the sexual cycle and fertility of females is limited. In this study, the effects of light pollution on the sexual cycle and the structural-hormonal characteristics of the thyroid and adrenal glands of female rats are investigated.
Methods: Twenty one female rats with regular sexual cycle were divided into three groups as control (12 h light/12 h dark cycle), experimental 1 (16 h light/8 h dark cycle) and experimental 2 (20 h light/4 h dark cycle). Animals were exposed to light for 47 days and vaginal smear was examined daily. Then, the animals were mated with male rats. Pregnancy was confirmed by observing the vaginal plaque of pregnancy. The pregnant animals were kept in the light/dark cycle conditions until the 19th day of pregnancy. On the 19th day of pregnancy, rats were bled under deep anesthesia and serum levels of T4, thyroid stimulating hormone and cortisol were measured. Then, thyroid and adrenal glands were dissected out and morphologically examined.
Results: In experimental groups 1 and 2, the sexual cycle became irregular and remained in the estrus stage in almost all rats. Compared with the control group, size of thyroid follicles decreased and the mean level of T4 hormone increased significantly (p < 0.05) in the experimental groups. Serum levels of thyroid-stimulating hormone and cortisol did not change, but the thickness of the fascicle area in the adrenal gland increased significantly (p < 0.05). 
Conclusion: The thyroid and adrenal glands of rats are sensitive to light pollution and their function is altered.‎