Melatonin and sunlight are intimately linked and the relationship is unique in the fact that there are two forms of melatonin, circulatory and subcellular. Both appear to be controlled by either the absence of sunlight or presence of sunlight. While circulatory melatonin may be the ‘hormone of darkness’ subcellular melatonin may be the ‘hormone of daylight’.
I’m happy to share a Melatonin Research article that my associate Scott Zimmerman co-authored with Prof. Russel J. Reiter. “Melatonin and the Optics of the Human Body“.
The main point of this review hypothesizes is that the subcellular melatonin is being produced in response to near infrared light (NIR) photons which make up the majority of natural sunlight.