It may be from cow brains but read further here.
Sources of Melatonin
Melatonin is present in all living organisms, including microalgae (green algae), bacteria, fungi, plants, small crustaceans (certain prawns and crayfish), fish, animals, and humans.163 Natural sources of melatonin, not standardized to provide a defined concentration, and with possible contaminants, also include medicinal plants such as feverfew (Tanacetum parthenium), St. John’s wort (Hypericum perforatum), and huang-qin (Scutellaria baicalensis),122,164 sometimes reaching levels of several nanograms per gram165 and possibly contributing to the therapeutic efficacy of the respective herbs.
High melatonin concentrations are found in seeds and some fruits such as tart cherries, bananas, and tomatoes.166,167 Melatonin also is found in food sources such as oats, rice bran, sweet corn, wheatgrass juice, and ginger. It has been shown that dietary melatonin (from plant sources) directly elevates the circulating level of melatonin in the body.
I doubt many of the products list hte source of the melatonin. It would certainly be less expensive to produce from plants.