California Poppy is a common plant in North and South America that has some traditional usage as a sedative and anxiolytic when made as a tea. Two rat studies have observed sedatory and anxiolytic actions, and one human study showed some promise to this claim but conclusions cannot be drawn since it also used another herb in conjunction with California Poppy.
In the preliminary stages of research, the unique alkaloid profile of California Poppy appears to interact with a variety of receptors in potent ways. The vast majority of these leads have not been further investgiated, and conclusions (especially on Serotonin) cannot be drawn at this point in time.
Currently, California Poppy is too preliminary to support any supplemental usage.