A couple of weeks ago, I was looking for some sort of newsy tidbit about circadian rhythms—the daily cycles that drive much of the physiology and behavior of living things—to share with my class of undergraduate students. My pick was a recent opinion piece by Kevin Gaston—an ecologist and conservation biologist in the UK—called “Lighting up the Nighttime” in the journal Science. From buildings to cars to street lights to signs—our attempts to reprogram the dark hours of our world into well-lit environments that allow us to use our eyes whenever and wherever we want and improve our sense of safety have come with some costs. Our own health may be at risk from shorter or disrupted sleep made possible or exacerbated by lighting up our nights. All those lights also require consuming energy resources to power the electricity. And, sadly, it is difficult or impossible to see the stars in the skies above well-lit urban areas.
Living in an urban area in North America, I have long appreciated the challenges of balancing our desire for nighttime lighting with maybe not being quite so disconnected from our nighttime skies. For example, the university campus where I work has an observatory. Nearby building, street, and sidewalk lighting has been an important consideration for those who would like a clear look at the skies above with a telescope. But good luck spotting many of the landmarks of the map of constellations by eye overhead anywhere in the city. I can reliably find Orion, the Big Dipper, and a few others, but the swath of the Milky Way and sheer number of stars are obscured by city lights. On cloudy evenings, the reflected glow from all those city lights means it’s never actually dark.
Troublingly, it appears we may have underestimated (or not considered at all) the impact that our nighttime lighting may have on other species that share our communities, especially insect populations, whose numbers are in steep decline for multiple (though incompletely understood) reasons. Insects provide food for birds and pollinate plants. They also represent a large swath of the diversity of species on the planet. A number of insect species are already targets of concern about light pollution (for example, bees, dung beetles, and fireflies, as profiled by the International Dark-Sky Association). In his article, Gaston argues that we need to seriously rethink how, when, and where we use outdoor lighting and what types of lighting may be least problematic moving into the future for these and other species. He offers specific suggestions for consideration, such as changing the timing or types of lights used.
How might all these lights actually be disrupting the lifestyles of living things? Many recent studies have looked at the effect of nighttime lighting on birds, especially in urban areas. For example, a 2013 study by researchers in Germany examined levels of the hormone melatonin in blackbirds. Melatonin is a hormone that helps regulate day-night rhythms and is perhaps most familiar for promoting sleep in humans. The researchers found that birds exposed to extra light at night had lower levels of melatonin. The birds also became more active earlier in the day, especially in winter. Birds use light cues to keep their circadian rhythms in sync. Changes in the patterns of activity due to artificial lighting could impact their singing, reproduction, and other behaviors.
What about nocturnal species? A study on bats published just this month by researchers in Illinois wanted to know what types of insects bats eat if they search for food near street lights or in darker areas. The researchers captured several different species of bats in lit and unlit areas and examined the DNA of their feces (!) for evidence of insect prey. Different bat species consumed different types of insects and showed different outcomes, so no clear-cut trend was evident. But, streetlights tend to attract moths and change the distribution of insects at night, so researching insect populations and the animals that feed on them is interesting. The researchers also considered what type of lighting was used; LED lights have different impacts on animals than older types of light bulbs. What those effects are is poorly understood and poses more questions than answers at this time. Clearly, more research is needed.
Meanwhile, many studies have already investigated the potential impacts of light at night on human health. Should we be using a computer or handheld screen at night? Does our own melatonin release change when we blast our eyes with light late at night? How does this impact sleep and other physiological rhythms? Does it matter what type of light it is? What about all that urban skyglow? Much evidence suggests that the blue wavelength of light may be particularly important for regulating our brains and bodies. A quick internet search will turn up endless articles on the topic from all types of sources. (For a thorough accounting of research on this area, check out this 2010 overview.) More and more, we are becoming aware that the very conveniences that help us maximize our waking hours may be impacting our sleep and biological rhythms that keep us healthy. Just last week, an article in the Washington Post demanded we take sleep and health more seriously with the headline, “Go to bed! Brain researchers warn that lack of sleep is a public health crisis.”
Perhaps as we start to rethink our relationship with sleep, our devices, and our 24-hour lives, we can also be more thoughtful about our use of artificial lighting. My grocery store has freezer units whose lights turn on automatically only when someone walks by. This saves energy used to power the lights. By analogy, do we need all our exterior lights all the time? What types of light bulbs and shielding are needed and best for our human goals and for wildlife? As we consider the needs of an increasingly urban society, these are interesting and important questions for communities to address from social, economic, energy, and ecological perspectives. We are starting to wake up to the power of darkness. I’m interested to see where this takes us.