Serotonin is the happy hormone, the chemical present in the brain that helps to control your mood and emotions. While serotonin is most closely associated with your feelings, it also impacts other biological functions like sleep, digestion, wound healing, and sexual arousal. The balance of this hormone is key to preventing mood disorders like depression and anxiety. In skin, serotonin is a precursor to melatonin and adequate Vitamin D is critical to balanced serotonin levels and the level of antioxidants available in the skin.