In recent years scientific studies have shown a link between light pollution and breast and colon cancers, perhaps because of the body's failure to produce certain light-related hormones.
Studies have also found that some people's immune systems are weakened by disrupted circadian rhythms, which are tied to natural light and dark cycles, Crawford says.
When it comes to safety at night, people often assume a brightly lighted area will help them steer clear of shady characters. Not so, Crawford says.
The bright lights increase the contrast between light and dark, giving criminals darker shadows for lurking. And overlighting can cause a blinding glare.
"Suppose you go to a grossly overlit gas station at night and get back on the road. You can't see for a while. You can't see what you're driving into," he said.
Excess lights are also a waste of energy and money, Crawford says. "I haven't noticed energy becoming cheaper lately."
"What we need is good light at night, not overlighting," Crawford added.
Good lights are those that shine toward the ground only, Brown, of Colorado State, says. When light hits the ground, its intensity decreases substantially and very little leaks back into the sky, he says.
In Tucson, where astronomy is big business, legislators enacted an ordinance that permits lights at night to shine only in a part of the spectrum that is easily filtered out by equipment on telescopes.
"That makes the extra ambient light almost insignificant to astronomers," Brown said.
But such lights are expensive and their use is controversial, Brown says. For example, one side effect is poor color rendition at night, making it difficult for police to distinguish between oil and blood on the street.
Nevertheless, Brown encourages cities, no matter their size, to at least adopt ordinances that prevent light leakage above the horizontal rim of the fixture.
"There's a whole heritage of the wonders of the night sky and all there is above to be seen and appreciated," he said. "That's what the average person has to lose if we lose the dark skies."
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