For most of human history, a person could look up at night and see an inspiring array of galaxies and constellations. Now, however, the glow cast by artificial light has masked out so much of the night sky that the Milky Way is invisible to 33 percent of the world's population. In some areas of North America, that total rises to 80 percent.
Light pollution also interrupts human circadian rhythms, innate 24-hour cycles that signal not only sleeping and waking periods but also eating, hormone release, physical activity, and many other processes. Cycles of light and dark help to keep circadian rhythms functioning on their proper natural schedules, and an excess of light during hours of expected darkness can cause measurable disruption. Such disruption may lead to a number of dangerous conditions, including sleep deprivation, mood disorders, and even diabetes and some types of cancer.
Light pollution endangers other animals as well. In the United Kingdom, scientists have concluded that excess light after sunset is causing spring to occur up to a week early, a change that can have a broad-reaching impact on various cycles in ecosystems, such as the budding of trees. Observers have also found that songbirds in Europe are beginning to change the songs that they sing at dusk and at dawn, while nocturnal species face risk to the navigational processes that help them to feed.