Pineal gland interface between the photoperiodic environment and the endocrine system.

The photoperiodic message that the pineal gland conveys to the organism is encoded in the circadian melatonin rhythm. Melatonin is a ubiquitously acting hormone that mediates seasonal changes in reproduction in nonhuman mammals and may have reproductive consequences in humans as well. Additionally, melatonin may relate to the function o f the immune system, hormone-responsive tumor growth, circadian rhythm disturbances, and a number of other processes.