Jul 17 · 6 min read
When the lights went out in Los Angeles after an earthquake in 1994, people called emergency centers to report a strange, giant, silvery cloud overhead. It was something they’d never seen before, something many humans alive today have not seen: the Milky Way, a swath of stars toward the center of our galaxy which, under totally dark skies in remote areas, graces the heavens like a river of silk. Without the bright lights of the city, many people experienced the natural phenomena for the first time.
Light pollution, or “loss of night,” as it is sometimes called, has only gotten worse since then, rendering all but a handful of stars invisible from big cities. But the effects go beyond frustrating backyard astronomers. Growing research suggests that bright outdoor lighting negatively affects sleep and moods in people young and old, and may even increase the risk of disease.
In places across America where artificial outdoor light is brightest at night, teenagers sleep on average 11 minutes less than teens in darker areas, according to a new study, published July 8 in the journal JAMA Psychiatry.
Researchers used satellite data to gauge light levels by census blocks (there are more than 200,000 of them in the United States) and compared that to answers from questionnaires given to 10,123 teens, ages 13 to 18. Teens in the brightest areas went to bed late and slept less, even after researchers accounted for population density. They were also more likely to have anxiety or mood disorders, including meeting the diagnostic criteria for bipolar disorder.
In places across America where artificial outdoor light is brightest at night, teenagers sleep on average 11 minutes less than teens in darker areas.
The study doesn’t prove that light is the only factor disrupting sleep. Light could be indicative of other issues, including noise.
“Although environmental light exposure is only one factor in a more complex network of influences on sleep and behavior, it is likely to be an important target for prevention and interventions in adolescent health,” says study leader Kathleen Merikangas, PhD, chief of the Genetic Epidemiology Research Branch at the National Institute of Mental Health.
The new research was based on data collected between 2001 and 2004, but “if anything, we would expect that our findings would be even more significant with the increase in environmental light” since then, Merikangas says in an email. And the effects probably aren’t limited to teens.
“It is likely that our findings extend to adults,” Merikangas says, “although youths may be more susceptible.”
The findings are no great surprise to experts. Inside of everyone is a biological clock, not quite as ancient as the stars but tuned across millions of years of evolution to the 24-hour cycle of day and night. The onset of dusk and then darkness triggers the release of melatonin, a hormone that helps us sleep. Bright light suppresses melatonin.
When properly fed by natural signals, this circadian rhythm, as it’s called, governs when we get tired and when we’re most alert. But the internal clock isn’t perfect — hence night owls.
And the natural signals are increasingly overwhelmed, as nights become less dark and many people spend less time outdoors, all in a very short time on the scale of human history. Not only are nights brighter than ever, but just in recent decades many people have come to spend far less time outside during the day compared to the entirety of human existence. Getting sunlight during the day is necessary to help set the biological clock, but a lack of that sun exposure plus bright indoor and outdoor lighting at night is a recipe for poor sleep, scientists say.
Not only are nights brighter than ever, but just in recent decades many people have come to spend far less time outside during the day compared to the entirety of human existence.
Further compounding the problem, human eyes are particularly sensitive to shorter-wavelength blue light, common in phones and other screens and increasingly common in street lights and parking lots across the country.
Older people in brighter areas of South Korea suffer more insomnia, as measured by the quantity of sleep aids they use, according to research published in the Journal of Clinical Sleep Medicine.
“This study observed a significant association between the intensity of outdoor, artificial, nighttime lighting and the prevalence of insomnia as indicated by hypnotic agent prescriptions for older adults,” said study team member Kyoung-bok Min, PhD, an associate professor at Seoul National University College of Medicine.
In a 2016 survey of U.S. adults, 29% of those living in the most light-polluted areas of the country said they were dissatisfied with their sleep, reporting an average of 412 minutes per night of shut-eye, compared to 16% and 402 minutes among people in darker locations. Those dealing with more light also reported higher levels of daytime sleepiness and more difficulty performing daily tasks and dealing with relationships.
“People may want to consider room-darkening shades, sleep masks, or other options to reduce their exposure,” said study leader Maurice Ohayon, MD, PhD, professor of psychiatry and behavioral science at Stanford University.
The impact of bright night lights may extend to health in surprising ways. Women exposed to high levels of light pollution have a 14% higher risk of breast cancer, according to a 2017 study in the journal Environmental Health Perspectives. Other studies have made similar connections to breast cancer and prostate cancer. While the studies found associations and not proof that light pollution causes diseases, some scientists theorize the suppression of melatonin could be directly responsible.
The American Medical Association (AMA) has for years warned about the health effects of the glaringly bright white LED street lights. Compared to other types of lighting, LED lights are more energy efficient, but they emit more short-wavelength blue light — the sort that’s more apt to keep us up at night. “White LED street lighting patterns also could contribute to the risk of chronic disease in the populations of cities in which they have been installed,” the AMA says.
On a more philosophical level, psychologist Dacher Keltner at the University of California, Berkeley, worries that the loss of the night sky robs humans of a sense of wonder that can “makes you feel like you’re part of something” and can translate into positive behaviors, National Geographic reports.
The International Dark-Sky Association encourages industry to develop lower-intensity, downward-facing lighting and works with cities and towns to adopt lighting policies that protect the night sky and even designate formal Dark Sky Places. The association suggests individuals can reduce outdoor light pollution by using “warm white” LED lights and shielding bulbs so they light only areas that are necessary.
Still, the loss of night appears to be increasing with population growth and urban sprawl, though the shift to LED lighting actually makes the phenomenon difficult to measure.
“More than 99% of the U.S. and European populations live under light-polluted skies,” according to a 2016 study published in the journal Science Advances. Light pollution is worse on cloudy nights, because upward-facing light is reflected back down to the surface.
“Near cities, cloudy skies are now hundreds, or even thousands of times brighter than they were 200 years ago,” says Christopher Kyba, PhD, who studies light pollution from the German Research Center for Geosciences.
Areas artificially lit at night across the planet grew by 2.2% per year, from 2012 to 2016, with the total amount of light, or radiance, increasing 1.8%, according to a separate study of satellite data published in 2017 in the same journal by Kyba and colleagues. In the United States, the amount of light measured was about the same across those four years, he says.
However, Kyba tells Elemental, the satellite used for the study can’t see blue light, which is visible to humans and which is common in white LED lighting.
“As people switch to white LEDs, the satellite ‘sees’ less of the light than it used to, so ‘staying the same’ in terms of the radiance observed by the satellite probably means ‘getting brighter’ in terms of what a human would see,” Kyba says.