Pu-erh ripe tea does not affect sleep primarily because of the "wo dui" (pile fermentation) processing technique. This high-temperature, high-humidity process nearly completely converts the caffeine (theobromine) and tea polyphenols in the tea leaves—substances that stimulate the nervous system and cause insomnia—into other compounds. Meanwhile, theabrownin and tea polysaccharides produced during fermentation actually have calming and stomach-warming properties.