"In most cities the sky looks as though it has been emptied of stars, leaving behind a vacant haze that mirrors our fear of the dark and resembles the urban glow of dystopian science fiction." —Verlyn Klinkenborg, Our Vanishing Night
Out of all the various pollutions humans create, light pollution gets the least attention. But light pollution is one of the most prevalent issues facing the developed and developing worlds today. Light pollution can come in several forms: light trespass is when unwanted light escapes from one property into adjacent properties; over-illumination is using excessive light where it isn't needed; light clutter is the redundant clusters of lighting found in many urban centers; sky glow is the collective light pollution found over big cities.
As stated in the National Geographic article Our Vanishing Night , light pollution is largely the effects of bad lighting design, which allows artificial light to shine outward and upward into the sky, where it's not wanted, instead of focusing it downward, where it is.
Common sources of light pollution include street lamps, neon signs and illuminated signboards.
Darkness is essential to our biological welfare. For centuries before the development of artificial light, human beings had become used to a day/night cycle of 12 hours of natural light and 12 hours of darkness. That cycle is a part of our circadian rhythms, an essential biological imperative which is dramatically affected by the presence of light at night.
According to an article in Sky & Telescope magazine , disruption of the circadian rhythm has been linked to sleep disorders like insomnia and delayed sleep-phase syndrome, as well as depression, hypertension, attention deficit disorder, obesity, diabetes, and heart disease.
Another important biological function that is disrupted by the presence of light at night is the production of melatonin. Melatonin is potent anti-oxidant, anti-carcinogen, and is responsible for regulating metabolism, and immune responses. According to a thesis presented to the University of California , less than 15 minutes of exposure to bright light at night can completely halt the production of melatonin. Lowered levels of melatonin have been shown to have correlation to to the rising rates of breast cancer amongst the developed world. According to another study, cited in the journal Environmental Health Perspectives , women who live in areas where it is bright enough to read a book outside at midnight had a 73% higher chance of developing breast cancer than women living in less brightly lit areas.
But humans aren't the only creatures affected by light pollution. Wherever human light spills into the natural world, some aspect of life for animals—migration, reproduction, hunting and feeding—is affected.
The most well studied incidence of this can be seen in the case of sea turtles. Sea turtle hatchlings instinctively know to head toward the brightest source of light, because for hundreds of years before mankind mastered the darkness, starlight reflecting off the ocean was the brightest thing on the beaches. But with coastal cities flooding the night with their bright lights, the hatchlings become confused and disoriented, heading toward the cities and not the sea, falling prey to dehydration and predation and never reaching safety of the ocean. Hatching losses in Florida alone number in the hundreds of thousands each year. 
While the environmental effects of light pollution are tragic, the economic effects can be just as shocking. The International Dark-Sky Association estimates that 1/3 of all lighting is wasted at an annual cost of $2.2 BILLION dollars. The light projected directly upwards from a cobra-head streetlight is about 30% of the total light it emits! To put that in perspective, the average desk lamp use 40 watts of electricity. To generate so much electricity, predominately coal-fueled power plants expel around 15 million tons of carbon dioxide pollution to the atmosphere each year.  So much air pollution results from the creation of electricity that fixing light pollution would be the equivalent of removing 9.5 million cars from the roads.
"Dark sky" does not mean "dark ground"; if you're smart, you can have a dark sky without losing light on the ground.
The most effective fix to light pollution is to turn OFF the lights! Otherwise use fully shielded light fixtures as they prevent light from being wasted outside of the space you desire to be lit.
Fully shielded light fixtures prevent light from traveling above the horizontal, stopping light from traveling up into the sky and greatly reducing sky glow. Research shows that 75% of artificial sky brightness comes from light escaping directly from fixtures, implying that replacing current fixtures with fully shielded one would reduce sky brightness to one-fourth of its current levels, not to mention the savings on your electrical bill.
Visit the John C. Wells Planetarium or darksky.org to find out what you can do to help reduce light pollution in your area! Searching for a movie? See THE CITY DARK!
Also, you can read the book, "The End of Night: Searching for Natural Darkness in an Age of Artificial Light," by JMU Assistant Professor of English, Paul Bogard. Published in July 2013 by Little Brown, the book blends personal narrative, natural history, science and history to shed light on the importance of darkness; what we've lost, what we still have, and what we might regain and the simple ways we can reduce the brightness of our nights.