Melatonin is a naturally occurring hormone made in the brain that helps regulate your internal clock of sleeping and waking times and is my favorite organic molecule. It is my favorite because I suffer from mild insomnia so melatonin is very important to me. This hormone is made in the brain by the pineal gland and is produced in higher amounts in the evenings and through the nights and then in the mornings, production slows (http://www.webmd.com/sleep-disorders/tc/melatonin-overview).
Melatonin is also called "the hormone of darkness" because production is increased due to darkness and light causes the production to slow. The hormone has also been used to treat seasonal affective disorder, sleep disorders, depression, cancer and other diseases and very interestingly can aid in increased fertilization in woman but is advised not to take as a supplement if a woman is trying to become pregnant (http://www.worldofmolecules.com/emotions/melatonin.htm).
It was Yale University researchers who first discovered melatonin at the end of the 1950s and realized that it was made in the pineal gland although the gland had been known of for thousands of years before (http://www.cancer.org/treatment/treatmentsandsideeffects/complementaryandalternativemedicine/pharmacologicalandbiologicaltreatment/melatonin).
It intrigues me that light levels can have such a large impact on sleep cycle regulation and the production of melatonin and that it what I like so much about this molecule. The formula for melatonin is C13H16N2O2 and this is its structure that I drew using Paint: