Melatonin

Melatonin is a hormone secreted in response to darkness. In the presence of light it's secretion is inhibited. Melatonin also has effects on the production of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the anterior pituitary gland. In long day breeders the increasing daylight hours cause a decrease in melatonin. In response, production of LH and FSH is uninhibited and so libido rises. In short day breeders an increase in melatonin causes a rise in production of LH and FSH and so leads to an increase in libido.

See also Seasonality of Reproduction