It’s December, and that means holiday lights everywhere -- strung along roof lines, windows, doors and trees. But all those festive lights aren’t good for the environment. They add to a bigger problem in both small and large cities these days: light pollution.

MORE: Read the rest od the December issue.

Artificial light pollution confuses nocturnal species, disrupts migratory birds and causes sleep disorders in humans, according to research. Studies suggest that the consequences of excessive exposure to light at night also increase the risk for obesity, diabetes, cardiovascular disease and cancer.

If left unchecked, artificial lighting could continue to grow rapidly, perhaps by up to 20 percent a year. Paul Bogard, author of The End of Night: Searching for Natural Darkness in an Age of Artificial Light, talked to Governing about the problem. This interview has been edited and condensed.

What is light pollution?

It’s the overuse and misuse of light. The primary problem is that we have a lot of light that is unshielded, which means that it’s allowed to go in all directions, including up into the sky where it doesn’t do anybody any good. Glare is a big problem. It makes it harder for us to see and it makes it harder for people to drive at night. Light isn’t the problem; it’s how we’re using it.

Has this worsened in recent years?

The estimates are that gas stations and parking lots are lit about 10 times as bright as they were 20 years ago.

We are at a crossroads right now with LED technology. The great thing about LEDs is that they have the potential to save money. The problem is that if you install an LED that is heavy in blue light -- they tend to be the cheapest -- it is the worst for human health and for other species in the environment. Blue light contributes the most to what we call “sky glow,” which is a type of light pollution. Blue-light LEDs increase sky glow by two to three times.

Are all LEDs a problem?

No. One of the great things about LEDs is that they are highly programmable; you can hook them up to your computer and you can program your whole city so that the lights are a little brighter at rush hour, and then at 3 a.m. you can turn them down and save energy. To save money, cities are choosing not to do that and are choosing to install the cheaper blue lights, so they are missing out on one of the huge benefits of LEDs.

What can cities and states do about light pollution?

Ensure streetlights are directed downward, and use shielded light [and motion sensors]. Several cities and states have laws about light pollution. What those ordinances are trying to do is set up lighting levels; they are basically trying to put into writing how brightly a place should be lit and how it should be lit. People are also trying to get shielded lighting into building codes. The International Dark-Sky Association is a clearinghouse for any government that is interested in doing this but doesn’t know how to get started.

Education would be huge, too. Lowe’s and Home Depot are both starting to stock shielded lighting for people’s houses. Lowe’s calls their lighting “good neighbor lighting.” So cities could do a better job of educating people by saying, “It’s unneighborly to light up, to trespass with light on your neighbor’s property. Keep your light on your property.”

Wouldn’t dimming the lights be less safe?

Just because some light at night can help us be safer and more secure doesn’t mean that ever-more light will make us ever-more secure. [It’s not safe to] have really bright lights shining, glaring into your eye, making it harder for you to see and casting shadows where the bad guys can hide. It creates an illusion of safety. The best lighting is uniform, low-level lighting.