Humans have been aggressively lighting up the night for just over a hundred years, and though scientists have spent only a fraction of that time exploring the impact of the unintended peripheral glow on the natural world, observational and experimental data show that it affects how animals move about, communicate, find food, and even select mates. The most famous example is newly hatched sea turtles that become disoriented by the light from brightly illuminated beach communities and have difficulty finding the ocean. But behavioral changes have been documented in a wide range of species, including birds, mammals, insects, reptiles, and amphibians.
Not all species respond to light in the same way. Many nocturnally migrating birds are drawn to the source and can wind up circling round and round lighted towers, often colliding with other birds or dropping from exhaustion. Some animals, such as mountain lions, avoid lighted areas at night; other species are able to exploit such areas—foraging longer or targeting prey that congregate near lights, as some bat species do. But the increase in foraging hours can have its downside, putting animals on the prowl at risk for predation. The overall effect on complex ecological relationships is not yet fully understood.
There is also evidence that our grand transformation of the night might have serious implications for our own health. The contrast between dark and light allows our bodies to calibrate our circadian rhythms, such as hormone levels and sleep schedules. Disrupting these can have a dramatic impact. Researchers think this phenomenon may help explain higher breast cancer rates in societies with brighter nights. Studies on shift workers exposed to nearly constant light during the night hours reveal a higher risk for the disease, perhaps because of altered levels of melatonin. Other research shows that blind women have a lower occurrence of breast cancer. One study that looked at a general population also found a correlation between neighborhood nighttime light levels and breast cancer incidence.
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