The benefits of dark night, a hidden common good

Nocturnal darkness, which defines the biological rhythms of organisms on our planet, is essential for life. But it is threatened by increasing light pollution.

It is one of the places of our ambivalence towards our environment. The dark night symbolizes our most archaic terrors, with all the monsters it could conceal in the recesses of its penumbra. But it also puts stars in our eyes. « The dark night is the source of Greco-Roman mythologies, of philosophical reflection on our place in the Universe, our origins », evokes Olivier Las Vergnas, president of the French Association of Astronomy.

This sensitive and metaphysical, universal experience, will it end up disappearing? To speak of the dark night is inevitably to immediately evoke light pollution, the misdeeds of light revealing the benefits of darkness. « In the middle of the Parc du Morvan and the Pic du Midi, 2,000 to 3,000 stars can be seen with the naked eye on a clear skycontinues Olivier Las Vergnas. In Paris, a maximum of thirty remain difficult to see. »

The areas from which the Andromeda galaxy, located 2 million light-years away, remains clearly visible are considerably restricted. In Europe and the United States, 99% of the population no longer live under starry skies, according to the International Dark Sky Association. « What is lost is the dimension of sharing the observation of the Big Dipper and Venus that everyone experienced in rural areas fifty years ago », continues Olivier Las Vergnas.

A good distance from a source of light pollution, if you have only given your eye the quarter of an hour it needs to adapt, a whole nocturnal universe is revealed. « Darkness offers a different relationship to nature, which also passes through the sounds of nocturnal fauna », recalls Samuel Challéat, researcher at the CNRS and coordinator of the Observatory of the nocturnal environment (Toulouse 2) who conducts research-action in connection with territories.

All life on Earth is organized around the alternation of day and night. « This is called the photoperiod, characterized by different light levels during the day and at night, explains Morgane Touzot, postdoctoral fellow at the Institute for Ecoscience at the University of Aarhus (Denmark). This alternation gives rhythm to the biological processes of animals, which synchronize with their environment, whether diurnal or nocturnal. »

And the biologist mentions the many studies demonstrating the deleterious effects of artificial light, which abolishes this distinction between day and night: sea turtles which, instead of heading at birth towards the sea where the moon is reflected, go towards street lights and car headlights; birds that exhaust themselves feeding their young in days that have become too long; hundreds of insects attracted by each lamp post at night instead of going to ensure nocturnal pollination and which will die for 30 to 40% of them from this fatal attraction; fish whose eggs no longer hatch, etc. « Light pollution has a real impact on biodiversity, through effects on movement, reproduction and even gene expression, with consequences for immunity,” alert Morgane Touzot.

Humans are obviously not exempt from these effects. In all mammals, melatonin, nicknamed the sleep hormone, is only produced by the body in the dark. Its reduction or elimination leads to an increase in fatigue, stress, anxiety and depression, as well as to diseases such as breast cancer, the incidence of which is, for example, higher in nurses who work at night. In his fascinating essay, Let’s dare the night. Manifesto against light pollution (Tana), the Swedish zoologist Johan Eklöf reviews the disturbances caused to trees by continuous light (early budding, foliage preserved late, etc.), which weakens them.

The nibbling of the dark night by artificial light, considered as progress, is not new, but it has been accelerating inexorably for several decades. « From the 1950s and 1960s, urban lighting favored by cheap energy was extended massively to extend the time of human activity, secure travel, goods and people and enhance heritage » , explains Samuel Challéat. The increase in light pollution is currently continuing at a rate of more than 2% per year. The International Dark Sky Association estimated before the current rise in the cost of energy that artificial light at night accounted for 20% of global electricity consumption. In the United States alone, at least 30% of nighttime lighting is wasted, costing $3.3 billion a year and emitting 21 million tons of CO₂.

Easy to adjust, light pollution? Easy, the return to the dark night? We press a switch and everything stops, unlike more complex pollution (chemical, sound, etc.)? If the implementation in a private garden is accessible, the fight against night lighting is a little more complicated beyond. “In practice there are problems of social acceptability, even if a transition is taking place in metropolitan France, emphasizes Sébastien Vauclair, president of the Dark Sky Lab design office. And we must not forget that, in many countries of the world, electrification continues and that the arrival of lighting in villages in Africa or elsewhere is perceived as a happy revolution. »