The HMPC (European Committee for Herbal Medicinal Products) has concluded that, based on its long-standing use, California poppy can be used to relieve mild symptoms of mental stress and to promote sleep.
The California poppy has long been used in folk medicine for its sedative properties. The California poppy contains a variety of plant compounds, including alkaloids. These compounds act on GABA receptors, which are distributed throughout the central nervous system. Human and animal research suggests that California poppy extract may have sedative and anxiolytic effects. A three-month study of 264 people suffering from mild to moderate anxiety showed that taking two tablets containing California poppy extract, hawthorn extract and magnesium twice a day reduced anxiety symptoms more effectively than a placebo (source https://pubmed.ncbi.nlm.nih.gov/14741074/).