Natural Medicines database does not usually have authors for their publications.
In this case, Natural Medicines database becomes the author.
Use last updated date on the bottom of the page for (date).
Natural Medicines. (2020, April 24). Melatonin [monograph]. http://naturalmedicines.therapeuticresearch.com
Note: No part of the Reference is italicized in this case.
(Author, Year)
(Natural Medicines, 2020).
Note: If the information is from same year, note as 2020a, 2020b, etc