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Cities are getting brighter every year.

Since Thomas Edison flipped the switch in 1879, electrical light has saved lives in hospitals, extended hours of play in parks, given us picturesque city skylines and allowed New York to become the city that never sleeps.

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But while the importance of artificial light is remarkable, the value of darkness is much less obvious.

Light pollution now blocks out the stars at night in many cities, and a growing number of city-dwellers have never seen the Milky Way. Canada’s biggest observatory, in Toronto, closed in 2008 because astronomers could no longer see a dark, starry sky.

Now scientists, environmentalists and some municipal politicians are sounding the alarm about light pollution for another reason: changes in human health and the ecosystem.

Light pollution has been linked to an increased risk of sleep disorders, cancers, diabetes, mental problems and obesity. It also has serious sometimes deadly consequences for animals who are being confused by artificial light at night.