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. So many factors can affect your quality of sleep, from when you eat dinner to blue light from screens.

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 lightbulb, 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. 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.