by Samantha Tracy
figures by Wei Wu

Walking down your neighborhood street you might notice the cars driving past, people waving hello, and the buildings lit up by the daytime sun. At night this view changes: we see fewer cars, and people have mostly returned to the comforts of their homes. But even as we shift from day to night, there is still a bright glow of light. After dark we continue to see light streaming from inside unoccupied buildings and illuminating empty streets. This use of artificial nighttime light gives us a wide view of our surroundings, but the glowing expanse of city lights limits our view of the night sky. 

As urban centers expand and the human population grows, night skies are threatened by encroaching urban light pollution. Recently, there have been increased efforts to understand the impacts of light pollution on human health. These efforts have focused on understanding the adverse effects of light pollution on sleep cycles and the potential implications on human health and chronic disease.

The current state of light pollution

Since the 1880s, the production of light technology has continued to develop rapidly. As the human population continues to grow and our urban centers become surrounded by suburban sprawls, the impact of artificial light on the night skies continues to increase. Current research estimates that over 80% of Americans are unable to see the Milky Way from their home residence. The glare from east coast cities is so intense that it is visible from space. Global scale light pollution studies estimate that 14% of the world’s population no longer use our evolutionarily adapted “night vision” when going outside after dark. Increases in light population, particularly around urban centers, means there are many people who have never seen a dark night sky (Figure 1). 

Recent studies have shown that global light pollution has increased by 2% per year between 2012 and 2016. This 2% growth rate is seen in both the amount of areas lit and the brightness of light. Light pollution increases have resulted from rising per capita light usage and inefficient lighting. Despite the efforts of some cities, like Flagstaff, AZ, to limit nighttime light, light pollution is continuing to encroach on the remaining dark sky areas. Scientists are just starting to understand the potential ecological consequences of pervasive light pollution and beginning to make ties to potential negative human health outcomes. 

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Figure 1: Expanse of light pollution around Boston, Massachusetts and relative night sky viewing capacity according to the Bortle Scale rating light pollution severity from a scale of 1 to 9.

Disrupted circadian rhythms impact human health

The increase of artificial light not only impacts our ability to see the night sky, but it also impacts our sleep-wake cycles. Sleep-wake cycles, or circadian rhythms, establish our biological clocks, allowing humans to react to the diurnal cycles. Circadian rhythms are universal patterns observed across bird, reptile, and mammal species. Regulated by the hypothalamus, a small region of the inner brain, circadian rhythms are essential for dictating critical information on daily and seasonal behavior patterns such as hibernation, eating, and breeding. The suprachiasmatic nucleus (SCN) is the part of our hypothalamus often referred to as the circadian pacemaker; it is responsible for sending time-based signals to regulate the body. 

The amount of light entering the eye is key in helping our brain distinguish times for wakefulness and times for sleep. The SCN is highly sensitive to light exposure, as light provides an external cue for internal clock regulation. Exposure to artificial light disrupts our internal clock regulation. Upon light exposure, the SCN starts a signaling pathway that leads to a reduction in melatonin, the hormone responsible for regulating our sleep-wake cycle. Melatonin reduction results in daytime sleepiness and sleep disturbances. It may also modify our immunity and stress response The amount of downregulation is dependent on the intensity and wavelength of light entering the eye. However, new research has found that even low light intensities from urban skyglow can cause melatonin downregulation.

Increased artificial light exposure has been linked with circadian rhythm sleep–wake disorders, where the circadian rhythm is misaligned with the natural light-dark cycle. Lack of melatonin and the disruption of healthy sleeping patterns is being increasingly linked to malignant cancers. Sleep pattern disruption is thought to interfere with cancer suppression genes, leading to an increased risk of breast, prostate, gastric, and lung cancers. 

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Figure 2: How light disrupts the circadian rhythm.

Circadian rhythm disruption has also been studied in wildlife, where light pollution has been shown to disrupt sleep cycles and mating habits and contribute to shorter life-spans. A study published this year has found significant negative associations between artificial light and insect biodiversity. Decreases in biodiversity, the number of different species within an ecosystem, is thought to be the product of interrupted foraging patterns, reproductive efforts, and predation mechanisms. Circadian rhythm disruptions have also been observed in avian species, and light exposure was found to impact the genes governing internal clock regulation. Similar outcomes have been discovered in humans as the distinction between day and night becomes more difficult to distinguish. 

The impact of indoor light pollution exposures on health

Outside, our streets and cities are flooded with lights, but this pattern of over-lighting spaces is also highly prevalent indoors. We now have more devices than ever that emit various types of light. Our smartphones and laptops stream out blue light, while our indoor lamps and overhead lights flood our spaces with white light. There is increasing evidence that indoor light exposure, particularly at night, can contribute to circadian rhythm disorders that have long term health impacts. Rats exposed to continuous lighting were found to have reduced skeletal muscle function and bone deterioration. Indoor lighting disrupts the SCN’s pathways in the same way as outdoor lighting. Circadian rhythm disruption and other negative health implications are likely occurring from increases in outdoor light pollution as well as increases in indoor light stimulation. 

How can light pollution exposures be limited

The disappearance of our night skies has largely gone unnoticed as the increasingly urban-based population has never had the opportunity to view a pristine night sky. Activist organizations, like the International Dark Sky Association, are working to preserve dark skies, as well as partnering with cities and municipalities to minimize the light pollution produced by electric light and improve energy efficiency. The good news is that light pollution is reversible. City ordinances and active community efforts have made major impacts in reducing outdoor light pollution and preserving night skies. 

There has also been increased advocacy to reduce circadian disruption from light infiltration via smartphones, computers, and indoor artificial light. Steps we can take to reduce our light pollution impacts include: switching light bulbs to warm-colored bulbs or LED lights, making active efforts to turn off unnecessary indoor or outdoor lighting, and protecting our eyes from blue light by using blue light glasses and screen protectors. These efforts not only improve night sky quality, but prevent negative health outcomes associated with increased indoor or outdoor light exposure. 


Samantha Tracy is a Masters of Science student in Environmental Health at Harvard School of Public Health. Her concentration is in Environmental Exposure Assessment. 

Wei Wu is a graduate student in the Design Studies program at Harvard University Graduate School of Design. Her concentration is Art, Design and the Public Domain.

Cover image: “A Night View Of Jaffna Street” by Muthulingam Tamilnilavan is licensed under CC BY-SA 4.0.

For More Information:

  • To learn about light pollution in your area, take a look at this interactive light pollution map
  • Look for a place to see a dark night sky, using this Dark Sky Finder
  • For a more scientific look at the health effects of light pollution check out this article in Environmental Health Perspectives
  • Check out The End of Night by Paul Bogard for a comprehensive read on the effects of light pollution

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