Humans

Even human beings are not immune to the effects of artificial light. As a species, humans are diurnal (which means that we are awake during the day, and sleep at night). Diurnal species feel safer in lit areas while they are awake. But physiologically, humans need a dark cycle too. During the night, in darkened conditions, humans produce melatonin, a necessary hormone that helps the body's cells repair themselves. Serious interruptions in the day/night cycle, such as shift work, have been demonstrated to interfere with melatonin production, resulting in a higher incidence of some cancers such as breast cancer.