Known by many names such as Tree of Collective Happiness, Silktree, mimosa, sleeping tree, tree of happiness, etc. Albizia julibrissin is a medicine from the Chinese materia medica that has gained popularity with western herbalists. And for good reason! It is useful medicine and it's naturalized in many parts of North America and Europe.
Western herbalists have used it as a relaxing nervine, good for increasing happiness, "herbal Prozac" as I've heard it called (along with st John's Wort). But if we think of it as "herbal Prozac" I think we might be disappointed with the results most of the time.
In Chinese Medicine, albizia 'calms the spirit' by relieving constrained liver qi. The concept of the Liver and constrained qi in Chinese Medicine are big topics, but in short we aren't talking about the anatomical liver. The Chinese Medicine Liver is a collection of anatomical systems and functions. One of it's main jobs is the free flow of qi and emotions. If this free flow of qi gets stuck, irritability and even anger can result, as well as other symptoms of stagnation such as discomfort, depression, and even digestive problems.
This can feel like pressure and stickiness in the chest, ribs, and upper abdomen, and insomnia, forgetfulness, anxiety and depression. Or that familiar feeling of feeling stressed out to the point of not being able to take a deep breath. Albizia is thought to act on the Chinese Liver to keep emotions flowing. One can see why this would help us happily live collectively.
I find it to be calming but not sedating and especially useful when emotions are stuck or irritation and anger have been present consistently. I mean, it looks like a pretty happy plant, right?