Feel Safe?  Nighttime exterior lighting provides security, right?  It depends. A study by Rutgers University found that common outdoor security lights often are less than useless.  A burglar forced to use a flashlight, or temporarily lit by a motion detector, is far more visible than one shielded by the glare of a security light.  In the 1970’s, one large urban public school system turned off its lighting on buildings and in parking lots. It found that not only were its electricity costs reduced, but vandalism also dropped dramatically.

Trees Suffer.  Here’s a surprising fact:  night lighting can harm trees and flowering plants.  Believe it.  A study by Purdue University noted that since the 1940’s it’s been known that the development mechanism of trees and flowering plants is influenced by the duration of uninterrupted darkness.  Artificial light effectively extends the night, promoting growth into early winter, when trees should be dormant. Winter cold can injure still-growing plants. The study notes that the red-infrared spectrum of incandescent lighting is particularly harmful.

The study also notes that trees subject to continuous nighttime light tend to grow more late foliage, making them more susceptible to water stress, air pollution, and winter icing.  

Wildlife, Too.  Another study found that nighttime lighting can disrupt a nocturnal animal’s biological clock, having adverse health effects.  As little as 10 to 15 minutes of exposure to moderate light, equivalent to twilight, can significantly shift the circadian rhythm of nocturnal animals.  One result is a reduction in production of hormones such as melatonin. Even low-intensity light can block melatonin production, adversely affecting the health of nocturnal animals.  Some epidemiologists believe that exposure to light at night reduces melatonin in humans, too, which may be the reason for increased cancers in populations which are subject to light for long periods, such as urban dwellers and night-time workers.

In addition, nighttime lights can be disorienting to animals that are trying to move at night.  Even a single light can compromise a wildlife corridor, preventing nocturnal animals from reaching important food supplies.  Of course, nighttime lighting also limits places animals can hide from predators.

Including Butterflies . . .  A third study examined the effect of nighttime lighting on insects.  It found that artificial lighting in natural areas has a significant impact on the diversity, distribution and abundance of butterflies.  A major part of the butterflies lifecycle is as a caterpillar, a creature often active only at night. Artificial lighting reduces the duration of the night, reducing the abilities of the caterpillars to reach food supplies, ultimately reducing the number of butterflies.  The same is true for moths. Fireflies, too, may not mate normally near incandescent lights. Many night-time insects are made more vulnerable by nighttime lighting.

. . . and Frogs.  Another study, cited in National Geographic, found that some frogs are silent in bright-lit areas.  If the males aren’t calling the females, the frog population will decline. That decline will result in a population decline of species dependent on frogs for food.  National Geographic noted that the amount of light to which frogs are exposed also affects the amount of fat they store for winter and when they produce eggs.

Just Flick the Switch.  National Geographic concluded that dark skies are a natural resource, worthy of protection just as an old growth forest or a scenic overlook.  But, unlike most environmental problems, there’s a simple solution. Quoting one researcher: “At the flick of a switch, this one could disappear.”