For at least a decade, the amount of artificial light shining at night has been increasing by nearly 10% a year. Light pollution hardly allows us to see the stars and makes the work of astronomers more difficult. Among the animals, male fireflies no longer find females in areas near cities, and there are fish that appear in tourist areas and approach the beach, believing that it is already daylight. But beyond the aesthetic value of viewing a starry sky, what are the effects of night lights on human health?
The journal Science is publishing a special issue on light pollution today, Thursday. The half-dozen articles published provide an overview of what science knows about the multiple effects. For example, the review notes that most of the planet’s telescopes can’t see the sky like they could a few years ago. He also mentions a paper published in 2020 that showed how artificial light disrupted animal life. One of the mechanisms of this disorder is related to a hormone that humans share with virtually all living things: melatonin. This hormone interferes with the biological clock, increasing its production and release at dusk and decreasing it as the day begins, initiating sleep or awakening.
The results of an experiment involving 100 people exposed to artificial light, published in December 2022, revealed the mechanisms of the link between excessive lighting and melatonin suppression. The body recognizes that night is approaching and it is time to sleep, above all by the eyes and the amount of light that penetrates through them. This is ensured by the photoreceptors in the retina. The best known are the cones and the rods (the former allow us to see the world in color, the latter work like night vision goggles). But there are other light-sensitive cells that were discovered a century ago but whose functions are only just beginning to be understood: intrinsically light-sensitive ganglion cells (ipRGCs), which are particularly sensitive to the blue part of the light spectrum. . By experimenting with these hundred people, we were able to confirm that both the cones and the ipRGCs contribute more than just vision. They are responsible for activating or short-circuiting the release of melatonin, thus influencing the circadian rhythm.
More information
The problem is that there isn’t much work analyzing the link between light pollution and human health. Most of these, like those of the hundred people, were carried out in the laboratory or with night workers, who were mainly exposed to indoor lighting. A higher incidence of cardiovascular diseases and cancers has been demonstrated in this population group. But only now are the effects of street lamps under real conditions known.
In one of the few experiments conducted, the results of which were published last year, motorists, pedestrians and people inside their homes were exposed to light from streetlights that have recently been converted to LED technology. In a series of experiments, they measured salivary melatonin levels. The work found no significant differences in melatonin suppression in the three groups. However, another study conducted in a New York borough in the summer of 2018, in which participants wore an activity bracelet, found a disrupted circadian rhythm. In the study’s conclusions, the authors write, “People who live in urban environments darken their days and lighten their nights.” The problem with both papers is that they were conducted with very small samples, one of 29 people and the others at 23, making it difficult to generalize their results.
Another influence of light pollution on the circadian clock that governs the day-night cycle goes back to a newly discovered pathway: the microbiome. It was known that the gut microbiota follows the circadian rhythm and is partly its protagonist, causing the abundance of certain species to increase or decrease depending on the time of day. Last year, researchers at National Taiwan University published a remarkable finding, albeit in mice: when exposed to artificial light at night, they found abnormal activity of ipRGCs cells (the retinal photoreceptors mentioned above) and an associated change in the rodent gut microbiota firmly. And there is growing evidence that human health largely depends on the state of their microbiome, particularly the gut.
Image of the Milky Way almost at dawn, photographed on La Palma in November 2022. Various studies have shown that stargazing helps in the treatment of depression.Egor Goryachev
ISGlobal researcher Barbara Harding is studying circadian rhythm disorders and their health implications in the group, led by the institution’s cancer program officer, epidemiologist Manolis Kogevinas. “We’re studying the effects of light pollution on cardiovascular health,” says Harding. The first results of the work, which have not yet been published, show that excessive light “has an impact on high blood pressure and obesity,” he says. The same group led another paper published in 2018 that linked excessive exposure to artificial light to some cancers, particularly blue light from LEDs.
One of the scientists who has studied light pollution most intensively is Professor Christopher Kyba from the German Center for Georesearch in Potsdam (Germany). Kyba is not an expert on health effects, but notes that there is “a lot of evidence that people who live in areas with brighter light tend to have poorer health outcomes, and for many of those (eg, breast cancer, metabolic syndrome) It is known that there is an association with exposure to light.” Kyba acknowledges that while light pollution has been shown to alter the lives of other living beings, “it has not been conclusively established whether outdoor lighting directly contributes to or causes these human health effects , rather than simply correlating with another factor also has implications.”
you can follow THE COUNTRY Health and well-being on Facebook, Twitter and Instagram.