Why Scientists Are Linking More Diseases to Light at Night

Oct. 22, 2024 — This October, millions of Americans missed out on two of the most spectacular shows in the universe: The northern lights and a rare comet. Even if you were aware of them, light pollution made them difficult to see, unless you went to a dark area and let your eyes adjust.
It’s not getting any easier — the night sky over North America has been growing brighter by about 10% per year since 2011. More and more research is linking all that light pollution to a surprising range of health consequences: Cancer, heart disease, diabetes, Alzheimer’s disease, and even low sperm quality, though the reasons for these troubling associations are not always clear.
“We’ve lost the contrast between light and dark, and we are confusing our physiology on a regular basis,” said John Hanifin, PhD, associate director of Thomas Jefferson University’s Light Research Program.
Our own galaxy is invisible to nearly 80% of people in North America. In 1994, an earthquake-triggered blackout in Los Angeles led to calls to the Griffith Observatory from people wondering about that hazy blob of light in the night sky. It was the Milky Way.
Glaring headlights, illuminated buildings, blazing billboards, and streetlights fill our urban skies with a glow that even affects rural residents. Inside, since the invention of the light bulb, we’ve kept our homes bright at night. Now, we’ve also added blue light-emitting devices — smartphones, television screens, tablets — which have been linked to sleep problems.
But outdoor light may matter for our health, too. “Every photon counts,” Hanifin said.
Bright Lights, Big Problems
For one 2024 study, researchers used satellite data to measure light pollution at residential addresses of over 13,000 people. They found that those who lived in places with the brightest skies at night had a 31% higher risk of high blood pressure. Another study out of Hong Kong showed a 29% higher risk of death from coronary heart disease. And yet another found a 17% higher risk of cerebrovascular disease, such as strokes or brain aneurysms.
Of course, urban areas also have air pollution, noise, and a lack of greenery. So, for some studies, scientists controlled for these factors, and the correlation remained strong (although air pollution with fine particulate matter appeared to be worse for heart health than outdoor light).
Research has found links between the nighttime glow outside and other diseases:
Breast cancer. “It’s a very strong correlation,” said Randy Nelson, PhD, a neuroscientist at West Virginia University. A study of over 100,000 teachers in California revealed that women living in areas with the most light pollution had a 12% higher risk. That effect is comparable to increasing your intake of ultra-processed foods by 10%.
Alzheimer’s disease. In a study published this fall, outdoor light at night was more strongly linked to the disease than even alcohol misuse or obesity.
Diabetes. In one recent study, people living in the most illuminated areas had a 28% higher risk of diabetes than those residing in much darker places. In a country like China, scientists concluded that 9 million cases of diabetes could be linked to light pollution.
What Happens in Your Body When You’re Exposed to Light at Night
Research has revealed that light at night (indoors or out) disrupts circadian clocks, increases inflammation, affects cell division, and suppresses melatonin, the “hormone of darkness.” “Darkness is very important,” Hanifin said. When he and his colleagues decades ago started studying the effects of light on human physiology, “people thought we were borderline crazy,” he said.
Nighttime illumination affects the health and behavior of species as diverse as Siberian hamsters, zebra finches, mice, crickets, and mosquitoes. Like most creatures on Earth, humans have internal clocks that are synced to the 24-hour cycle of day and night. The master clock is in your hypothalamus, a diamond-shaped part of the brain, but every cell in your body has its own clock, too. Many physiological processes run on circadian rhythms (a term derived from a Latin phrase meaning “about a day”), from sleep-wake cycle to hormone secretion, as well as processes involved in cancer progression, such as cell division.
“There are special photoreceptors in the eye that don’t deal with visual information. They just send light information,” Nelson said. “If you get light at the wrong time, you’re resetting the clocks.”
This internal clock “prepares the body for various recurrent challenges, such as eating,” said Christian Benedict, PhD, a sleep researcher at Uppsala University, Sweden. “Light exposure [at night] can mess up this very important system.” This could mean, for instance, that your insulin is released at the wrong time, Benedict said, causing “a jet lag-ish condition that will then impair the ability to handle blood sugar.” Animal studies confirm that exposure to light at night can reduce glucose tolerance and alter insulin secretion — potential pathways to diabetes.The hormone melatonin, produced when it’s dark by the pineal gland in the brain, is a key player in this modern struggle. Melatonin helps you sleep, synchronizes the body’s circadian rhythms, protects neurons from damage, regulates the immune system, and fights inflammation. But even a sliver of light at night can suppress its secretion. Less than 30 lux of light, about the level of a pedestrian street at night, can slash melatonin by half.
When lab animals are exposed to nighttime light, they “show enormous neuroinflammation” — that is, inflammation of nervous tissue, Nelson said. In one experiment on humans, those who slept immersed in weak light had higher levels of C-reactive protein in their blood, a marker of inflammation.
Low melatonin has also been linked to cancer. It “allows the metabolic machinery of the cancer cells to be active,” Hanifin said. One of melatonin’s effects is stimulation of natural killer cells, which can recognize and destroy cancer cells. What’s more, when melatonin plunges, estrogen may go up, which could explain the link between light at night and breast cancer (estrogen fuels tumor growth in breast cancers).
Researchers concede that satellite data might be too coarse to estimate how much light people are actually exposed to while they sleep. Plus, many of us are staring at bright screens. “But the studies keep coming,” Nelson said, suggesting that outdoor light pollution does have an impact.
When researchers put wrist-worn light sensors on over 80,000 British people, they found that the more light the device registered between half-past midnight and 6 a.m., the more its wearer was at risk of having diabetes several years down the road — no matter how long they’ve actually slept. This, according to the study’s authors, supports the findings of satellite data.
A similar study that used actigraphy with built-in light sensors, measuring whether people had been sleeping in complete darkness for at least 5 hours, found that light pollution upped the risk of heart disease by 74%.
What Can You Do About This?
Not everyone’s melatonin is affected by nighttime light to the same degree. “Some people are very much sensitive to very dim light, whereas others are not as sensitive and need far, far more light stimulation [to impact melatonin],” Benedict said. In one study, some volunteers needed 350 lux to lower their melatonin by half. For such people, flipping on the light in the bathroom at night wouldn’t matter; for others, though, a mere 6 lux was already as harmful — which is darker than twilight.
You can protect yourself by keeping your bedroom lights off and your screens stashed away, but avoiding outdoor light pollution may be harder. You can invest in high-quality blackout curtains, of course, although some light may still seep inside. You can plant trees in front of your windows, reorient any motion-detector lights, and even petition your local government to reduce over-illumination of buildings and to choose better streetlights. You can support organizations, such as the International Dark-Sky Association, that work to preserve darkness.
Last but not least, you might want to change your habits. If you live in a particularly light-polluted area, such as the District of Columbia, America’s top place for urban blaze, you might reconsider late-night walks or drives around the neighborhood. Instead, Hanifin said, read a book in bed, while keeping the light “as dim as you can.” It’s “a much better idea versus being outside in midtown Manhattan,” he said. According to recent recommendations published by Hanifin and his colleagues, when you sleep, there should be no more than 1 lux of illumination at the level of your eyes — about as much as you’d get from having a lit candle 1 meter away. And if we manage to preserve outdoor darkness, and the stars reappear (including the breathtaking Milky Way), we could reap more benefits — some research suggests that stargazing can elicit positive emotions, a sense of personal growth, and “a variety of transcendent thoughts and experiences.”
SOURCES:
John Hanifin, PhD, associate director, Light Research Program, Thomas Jefferson University.
Randy Nelson, PhD, neuroscientist, West Virginia University.
Christian Benedict, PhD, sleep researcher, Uppsala University, Sweden.