For most of human existence, daily life revolved around the sun’s schedule. People were active during the light of day and rested at night. But that’s no longer true in much of the world. In the United States, as much as two-thirds of the population cannot see the Milky Way at night. That’s because man-made light in cities, towns and the suburbs mutes the dark sky above. A growing body of research indicates that exposure at night to artificial light is causing problems for sea turtles, birds and other creatures — as well as humans. For this month’s Environmental Outlook, efforts to combat light pollution.