WASHINGTON -- Are the streetlights and security lights the Western world takes for granted causing breast cancer, killing sea turtles and blocking views of the constellations?

Last week scientists said: Well, maybe.

"There are a lot of people in the world who don't know the difference between night and day," said David Crawford, an astronomer and co-founder of the International Dark-Sky Association, at a conference on nighttime light held here.

More than 100 scientists, lighting technicians and government workers registered for the two-day conference hosted by the Carnegie Institution, a non-profit research center.

George Brainard, a professor of neurology and pharmacology at Jefferson Medical College in Philadelphia, said some research points to a rise in breast cancer for women living in industrialized countries.

The bombardment of light may prevent the brain from producing melatonin, a hormone some researchers think combats cancer, Brainard said. "Melatonin follows the pattern of dark time," he added.

But Brainard didn't declare a direct link between breast cancer and streetlights. He said additional research is needed to confirm that "if light can be beneficial, then if it's not used correctly, it can harm."

Crawford, whose organization planned the event, "The Night: Why Dark Hours Are So Important," said the growing number of bright lights stretched across the world obscures the circadian rhythm, the 24-hour cycle on which most organisms operate.

He described the problem as "obvious" because total darkness no longer exists in many places, deterring most humans and animals from their natural processes, such as adequate sleep.

"Do you see the glare and the clutter? Why do they do that?" he asked the crowd inside the dark auditorium where the meeting took place.

Lorna Patrick, a biologist with the U.S. Fish and Wildlife Service in Florida, is concerned about baby sea turtles, which she said face death when streetlights neighbor beaches.

Upon hatching, the young head for the ocean because usually it's the brightest light source, she said. But that has changed.

"We have found dead hatchlings at the bottom of streetlights," she said.

That change is one of many new problems facing animal life as lights get brighter.

"We don't really know what will happen, but there could be an impact on population," said Bryant Buchanan, who studies frogs and salamanders at Utica College in New York. He said his research found changes in the growth of frog larvae that received too much light. The larvae produce melatonin during darkness, just like humans.

As for the constellations, the stars and planets, earth light might one day block them, too.

Some scientists said future generations may see them only in textbooks.