Light at night: Mixed blessing!

Print Friendly
Light at night: Mixed blessing!

But with the steady advance of lighting, we may be the last generation to know that. Whether seen from ground or space, lights glare from the cities and suburbs, brightening the places we live, and spilling over into the places we sleep, and where the animals live.


Satellite image of Europe at night shows brightest lights in the West

Credit: NASA
Seen from space, Europe’s cities and highways look as bright as day. How does that affect animals that have evolved with a dark night and a bright day?

For decades, astronomers have moaned that “light pollution” was wrecking their view of the universe. Now, ecologists and medical experts are wondering how this extra light affects our world and our health.

The evidence is disturbing. For example, studies have linked light at night to human cancer and obesity, while other researchers have measured significant changes in animal behavior. Birds, for example, start their “dawn song” hours before they would normally sing. Although bright nights help predators hunt, their prey don’t get around much anymore and therefore have less time to feed, cutting their ability to reproduce.

Estimates say that light pollution is rising as much as 20 percent per year in some locations, with an average of 6 percent.

Expect more change to come. It’s not just overall brightness that is growing in cities: The energy-saving light-emitting diodes (LED) now being installed are bright in the blue end of the spectrum. Blue wavelengths severely inhibit the release of melatonin, a hormone that controls the daily metabolic cycles called the circadian rhythm.

Even the laptops and tablets that we use to read in the evening probably reduce melatonin secretions, since their displays are rich in blue light.

Bright white night

One major aspect of night lighting that has changed with the surge in electric lighting is the role of clouds. Christopher Kyba of the Free University of Berlin says they once made dark nights even darker, but now they reflect city light back to the surface. In Berlin, Kyba found a 10-fold increase in illumination on a cloudy night1.


Left: Very starry night in the mountains; right:  bright, cloudy night sky in the city; no stars visible.

Credit: Left: © Ray Stinson, Glacier National Park, USA; right: © Christopher Kyba, Berlin.
Left: clouds over Glacier National Park form dark patches against the brilliant starlit sky. Right: “sky glow” brightens Berlin on a cloudy night.

As scientists explore the biological effects of lighted skies, “cloudy nights are so much brighter and hence more important,” Kyba says. “In evolutionary times, it was always the case that clouds made the sky darker; we have the phrase, ‘a dark and stormy night.’ In the last century, this has completely reversed.”

So how is nighttime light affecting the environment, and its human occupants?

Night light plight: the human side

Light at night is one of the great blessings of the electric age, making cities safer and nights more rewarding, enjoyable and profitable, and loosening the link between sundown and bedtime. With so many obvious benefits, the question of drawbacks was slow to arise, even as the number and intensity of light sources exploded.


Image shows sleep deepest at 2 am, other changes through the 24-hour cycle.

Credit: Wikipedia
Circadian (24-hour) rhythms affect many aspects of human (and likely animal) biology. Body temperature, sleep, alertness, coordination and blood pressure are all part of the circadian cycle.

The hormone melatonin, produced by the pineal gland in the brain, is crucial to the “body clock,” a complex timing mechanism that makes us warm, awake and alert during the day, and cooler, drowsier and more relaxed at night. Disturbances to the body clock cause jet-lag, and are implicated in night-time accidents and a number of diseases.

Melatonin also can fight breast and prostate cancers, says Richard Stevens of the University of Connecticut. Stevens, who proposed in 1987 that light at night might play a role in the breast cancer pandemic2, says, “We thought it was related to diet. That turned out not to be true, unfortunately … but what else had changed? One thing was electric light at night.”


Two cops walk at night; one is yawning

Night-shift work is notoriously hard on the body; largely because it de-programs the body clock. Obesity and cancer rates are high among shift workers.

Although light inhibits the release of melatonin, the possible linkage was not taken seriously for years. But then studies of night-shift workers showed excess breast cancer. The International Agency for Research on Cancer says, “shift-work that involves circadian disruption is probably carcinogenic to humans.”

Although light is a key to setting or upsetting the circadian rhythm, other factors, such as sleep and eating patterns, also play a role in our chronobiology.

A second prediction was that breast cancer should be more common in places where nights are brighter, and that correlation has held, first in Israel3, and in 2010, globally 4

The same correlation surfaced in prostate cancer: the rate of this male cancer was 110 percent higher in nations with the brightest night lights — typically in Europe and North America — compared to the darkest countries.5

Night light, a slight blight?

Still Stevens cautions that these studies do not prove that light at night causes cancer: “The evidence is necessary, but not sufficient,” to prove a large effect. Conversely, had the study found no correlation, “that would be good evidence against a large effect.”

So far, the theory that light at night “causally increases the risk of breast cancer” has survived several challenges, Stevens says. “We have made predictions, and so far, the evidence is all aligning in the same direction.”

It’s not just epidemiology that’s linking light and the body clock:

A 2005 study showed that blood taken from women exposed to light at night supported human breast tumors growing in mice. These tumors were inhibited by melatonin-rich blood taken from women who slept in the dark6

Satellite photo before the Northeast blackout of 2003 United States plunged millions into darkness. ROLLOVER image: Many cities disappeared during the blackout.

Credit: Aug. 15, 2003, NOAA/DMSP

A recently-recognized light-detecting cell in the retina alerts the brain to a wavelength of blue light that’s most intense in the morning. This cell uses melanopsin to detect light. It’s not involved in vision, but seems to be the missing link between light and the body clock.

In lab experiments, room light before bedtime delayed and shortened melatonin release7. Thus evening light could affect not only sleep, but body temperature, blood pressure and glucose metabolism.

Research by Eve Van Cauter of the University of Chicago links circadian disruption and obesity. When the sleep of volunteers was disrupted for a week, their blood had lower levels of the hormone leptin in the morning. Leptin is a chemical signal sent by fat cells that tells the body to stop eating; a low leptin level is essentially a hunger signal. But although the study focused on sleep disruption, it also exposed the subjects to light when they awoke at night, Stevens says.

More evidence for metabolic links appeared in a 2011 review8: “epidemiological and experimental data support associations between disrupted physiological rhythms, a reduction in adequate sleep, and light-at-night-induced suppression of … melatonin.” Treating these problems with life-style changes could help control weight, the authors concluded.

The oldest clock?

“Dim is beautiful” is one reasonable response to all this emerging information, says Stevens. “I am not giving up electricity. I’m not saying all light at night is bad. But I think it’s best to get as dim as you can a couple of hours before bedtime.”

Spectacled man sits in dark, stares blankly at computer screen which lights up his face.

Credit: ob79
Laptops and other digital thingies bring the world to your fingertips — but how do they affect organisms that evolved in a never-ending cycle of day and night?

Broadly speaking, the wide biological implications of light and time of day are more evidence that we have evolved in a world dominated, and calibrated, by the sun. “The circadian clock controls our lives!” writes Martha Merrow of the University of Groningen in the Netherlands. 9 “It dictates when we sleep and wake and when we perform and think optimally.”

The study of what’s called “chronobiology” may be new, but responding to daylight and darkness is almost as old as life itself. “Clock genes,” which have 24-hour rhythms that run largely independent of outside input — are ubiquitous in our cells.

“The clock in our brains – and that in each of our cells – is not so different from clocks described in mice, fruit flies, plants and algae, fungi, and even some bacteria,” Merrow adds.

That list includes cyanobacteria, the single-celled organisms that oxygenated the planet and originated as much as three billion years ago.

If, as Stevens says, “The strongest cue to our physiology is light,” we’re nervous, given how many people read magazines and blogs, or watch TV and movies, on an eye-pad or laptop before bed.

These devices, after all, are rich in the blue light that is so destabilizing to the body clock.

Night light in the realm of fight or flight

When wildlife scientists worry about the glare of light at night, they mostly think about street lights and sky glow — the gauzy cloud of brightness that envelops our cities. The growing concern is not surprising, since 64 percent of mammals are nocturnal, and lighting even affects fish. A study of Atlantic salmon in England found that street lighting disrupted their tightly scheduled migration10.

Credit: Data from F. Hölker11
A surprising number of animal species are nocturnal. (However, light pollution can also extend the activity of diurnal species, which are active in the day, and change the behavior of predators and competitors.)

The ecology of light today

Last week’s meeting of the Ecological Society of America provided more evidence for the ecological implications of light pollution.

Light changes desert mouse behavior

Scientists in Israel exposed two mouse species, one nocturnal and one diurnal, to three hours of artificial light, starting at dusk. As predicted, the nocturnal mouse cut its foraging time, especially in exposed locations. The shift could increase competition among the mice and open an ecological niche to exotic species.


Small gray mammal sits at base of tree and fence.

The opossum, the only marsupial in North America, climbs trees and scavenges for food at night. Will light pollution help their predators hunt?
Street lights affect bird reproduction

A European study showed that males in four of five bird species started singing earlier in the morning near street lights, and copulated twice as much with non-mates. “… light pollution has substantial effects on the timing of reproductive behavior and on individual mating patterns,” the scientists wrote. Normally, males, especially older ones, sing early in the morning to signal reproductive fitness and defend territory, comments Travis Longcore of the Urban Wildlands Group. “But males closer to the lights, even though they were younger, were singing earlier, and attracting females, and getting extra copulations. … Lightning is disrupting a previously reliable signal of fitness, which can affect evolution.”


Lit up city skyline at night and orange sky, Empire State and Chrysler Buildings are visible.

New York City’s skyline against the sky glow. It’s easy to believe that this intense light pollution would affect wildlife.
Office buildings: Snare for aquatic insects?

Insects are major ecological players, and a 2006 study from Zurich, Switzerland, measured the impact of building lights on insects at an office building near a stream. When the windows were lit, sticky traps — think flypaper — caught up to several thousand insects per square meter, compared to at most 60 when the windows were unlit. Although these insects would not die (unless they were trapped on scientific flypaper!), the scientists wrote that “disorientated insects lose time, energy and are exposed to predators.”

Comparing different lights

In 2011, experimenters in the Netherlands erected 120 all-night lights at the edge of forests. Some lights shine white. Others lack short wavelengths and make red light. The third group lack long wavelengths, producing green. During the four-year experiment, specialists will study breeding birds, bats, other mammals, moths and vegetation. The study’s rigorous controls improve on existing observations, says Longcore, co-editor of a pioneering book on the ecology of light12. “They are doing manipulative experiments to really understand what adding light does to an ecological community, separate from everything else.”

Forest with line of lamp posts shine red light.

Courtesy Kamiel Spoelstra, Netherlands Institute of Ecology
This outdoor experiment shines white, green or this eerie red light, which is deficient in the short wavelengths that affect circadian rhythms. If red or green is less harmful than white, street lights could be “tuned” to benefit wildlife, though the bizarro color scheme might take some getting used to!

Taking the measure of light

Overall, the biology of light is moving toward greater precision, says Longcore. “We need more quantification of natural light regimes, to measure the spectrum and intensity over the course of a night, with and without the full moon. This will help us understand the effect of sky glow and connect that to biological phenomena.”

As light pollution threatens to “eliminate species that specialize in the dark,” Longcore says data on the different colors of light helps us “understand that certain behaviors are taking place at certain illuminations.”


Five cylinders side by side, zip-tied to a machine; all set on a roof.

Courtesy Christopher Kyba
Filters on a commercial light-measurement device are conducive to widespread monitoring of light at night, filling in essential details about which wavelengths are present, and at what levels.

If light’s biological effects result from its intensity, timing and wavelength, then we need to know more about the light we’re living with. That’s particularly true as older street lights are being replaced with LEDs, which have higher energy efficiency and produce a vastly different spectrum of light.

Kyba says the last transformation of city light, following the adoption of mercury and sodium-vapor lights about 40 years ago, went undocumented. “This changed the whole country, but we had no record of that happening. Now we are on cusp of a new technology, and hopefully we will have more monitoring in place, that includes the spectral information.”

Because LEDs can be tuned to intensity, wavelength and direction, better data on effects can help cities and citizens choose lighting that meets legitimate needs without overwhelming animal and human retinas, Kyba says.

In sight: light right

LEDs can be positioned to illuminate streets and sidewalks without shining into bedroom windows or up to the sky. “They are dimmable, so you could run them at 100 percent until midnight and then dim them, or have motion sensors, so they brighten when someone walks by,” Longcore says. “There is potential to save a lot of electricity and carbon emissions, and still have reasonable lighting. But they can be installed crappily, with no thought to where the light is going” or what type of light is best for the situation.

As the ecological impacts of light pollution become clearer, the details will allow for improvements. For example, the Urban Wildlands Group is working on wildlife-friendly lighting guidelines for the National Park Service. Says Longcore, “We are moving beyond acknowledging what is going on to figuring out ways to get the amount of light when it is needed, how it is needed for safety and the visitor experience, while minimizing the impacts on wildlife.”

– David Tenenbaum

Terry Devitt, editor; S.V. Medaris, designer/illustrator; Molly Simis, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive


  1. Kyba CCM et al, (2011) Cloud Coverage Acts as an Amplifier for Ecological Light Pollution in Urban Ecosystems. PLoS ONE 6(3): e17307. doi:10.1371/journal.pone.0017307
  2. Electric light causes cancer? Richard G. Stevens, Mutation Research 682 (2009) 1–6
  3. Kloog I, et al, 2008, Light at night co-distributes with incident breast but not lung cancer in the female population of Israel. Chronobiol. Int. 25:65–81.
  4. Kloog I, et al, 2010, Nighttime light level co-distributes with breast cancer incidence worldwide. Cancer Causes Cont. 21:2059–2068.
  5. Global Co-distribution of light at night and cancers… in men; Itai Kloog et al, Chronobiology International, 26(1): 108–125, (2009)
  6. D.E. Blask et al, Melatonin-depleted blood from premenopausal women exposed to light at night. Cancer Res. 65 (2005) 11174–11184
  7. Exposure to Room Light before Bedtime Suppresses Melatonin Onset …, Joshua J. Gooley et al, J Clin Endocrin Metab, 2010, doi:10.1210/jc.2010-2098
  8. Obesity and metabolic syndrome: Association with chronodisruption …, Russel J. Reiter et al, Annals of Medicine, posted online on June 13, 2011, (doi:10.3109/07853890.2011.586365)
  9. FEBS Letters 585 (2011) 1383
  10. W.D. Riley et al, Street lighting disrupts the diel migratory pattern of wild Atlantic salmon… Aquaculture, (2011) doi: 10.1016/j.aquaculture.2011.12.009
  11. Light pollution as a biodiversity threat, F. Hölker et al, Trends in Ecology and Evolution 25(12):681–682, 2010.
  12. Ecological Consequences of Artificial Night Lighting, Catherine Rich and Travis Longcore, Island Press, 2006
  13. Red is the new black: how the colour of urban sky glow varies with cloud cover, C. C. M. Kyba et al, Mon. Not. R. Astron. Soc. (2012), doi:10.1111/j.1365-2966.2012.21559.x
  14. Light at night linked to obesity
  15. Selling LED lights
  16. Lighting impacts research agenda
  17. Our Vanishing Night (video)
  18. Blackout! Power surge blacks out Northeast
  19. Final report on 2003 blackout in U.S. and Canada
  20. Effects of Artificial Light at Night on Wildlife
  21. Night shift workers, light at night, and cancer