Some vitamins are toxic if taken
in excess as well, big-time, and others, are peed out when taken
in excess. The major ones to be concerned with in excess are A, D, E, and K. Again, I could list all vitamins and minerals if you want.
Melatonin: I don't know all there is to know about this, but I can tell you that it is a hormone that is produced by the pineal gland and is produced at night. It is thought to to influence sleep-wake cycles, and be involved in the onset of puberty.