Melatonin is just for jetlag/otherwise shifted sleeping schedule, are people using it as an anti aging drug?
Serotonin is made from tryptophan, which can also be made into niacin.
Taking tryptophan and niacin as a supplement will boost both serotonin and melatonin naturally, assuming these pathways are working properly.
However, this can be dangerous in the presence of an SSRI.