Melatonin

Melatonin is a hormone which is an indolamine derivative of serotonin secreted from the pineal gland into the blood and CSF 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. The purpose of melatonin in many mammals appears to be the control of physiologic rhythms by altering the timing of circadian rhythm data relayed from the suprachiasmatic nucleus in the hypothalamus. Melatonin may inhibit dopamine, reduce free radical production and central nervous system excitability as well as augmenting GABA.