Ok Im a psyc major. I will like to begin by letting you know teir are different types of depressions.
Firts off it is possible their is a depression gene but it isnt known yet for sure; next you can become depressed by the use of drugs that mess with your seratonin and melatonin. For instance everytime lets say you do extacy not saying you do, you loose more melatonin permanently meaning you are more depressed permanently.
Eventhough the Brain gives off more melatonin in the mornin(for you to wake up) and more seratoning at night (so you go to bed) it doesent mean the brain is making more seratonin or more melatonin it just means its using what it has when it needs it.if you take the melatonin away completely you would commit suicide
That right there is the basics. If you want to know about specific depressive dissorders, then I suggest you see the DSM-IV revised edition.