Originally Posted by reverett123

Melatonin is produced by the pineal gland. It is probably more a question of what is the pineal gland since melatonin is its messenger. The PG is a central part of the time-keeping system of our bodies and seems to be regulated by light by way of signals from the retina. When the retina sees light the PG stops making melatonin and resumes in the dark.

But, like much of the endocrine system, we just don't know. Until relatively recently it was considered to be one of those "vestigal organs" that we seemed to have several of.