The mechanism of action of stellate ganglion block has generally been explained by vasodilation within its sphere of innervation. However, the success of treatment cannot always be explained by just one mechanism of action, because its clinical indications in Japan extend to many diseases, including systemic diseases. We propose a new mechanism of action for stellate ganglion block that is based on correction of melatonin rhythm disorder resulting from increased sympathetic nerve tone and does not involve vasodilation.