Evidence suggests that some people require more UV light exposure in order to function properly. Bright light, such as mid-day summer sun, causes your brain to stop the release of melatonin. Melatonin acts as a depressant in the body if it's generated during the daytime. Effected people, exposed to longer hours of bright light, feel happier and enjoy life more. Bright light sources that emit only visible light are commonly used to treat Seasonal Affective Disorder or SAD.