Melatonin, Serotonin, and Tryptophan... Check out the info on line. Tryptophan is easy enough to get in turkey and milk. Melatonin is created by your brain when sitting in the dark (this is why people. Who get snowed in on a regular basis deal w/ depression. They feel tired and worn down after spending a great deal of time indoors away from sunlight.)
The human body creates Serotonin in reaction to tryptophan. As I said before, it CAN cause depression. Melatonin is not known so much for causing depression, as it is know for having a calming effect. I recommend Melatonin, if you're going to "self diagnose." If you're working w/ a doctor, they might be able to help you get on the right dosage of either one.
Hope this helped.