In towns and the countryside alike, it’s not really dark anymore at night. Blame it on street lighting, of course, but also on the advertising panels, shop windows, illuminated monuments, stadiums, shopping areas, ports and airports, car headlights, homes… If they seem both practical and beautiful, all these light sources have an impact on us humans, and on various ecosystems.

Light or Night

New York, August 14, 2003. When a gigantic power cut paralyzed the city for several hours, New Yorkers prepared to spend a night in complete darkness. In the evening, emergency services received an unusual number of worried calls informing about the appearance of a luminous trail in the night sky. This phenomenon was, in fact the Milky Way, the galaxy in which our planet resides. Usually, the Milky Way is invisible to the eyes of the New Yorkers because of the halo of light blocking the sky.

It's a fact: over 80% of the world's population live under a polluted sky by human-made lights , as evidenced by this website which tracks the evolution of light pollution over the years (https://lighttrends.lightpollutionmap.info/). Experience it: go for a walk around three in the morning (yes, do it), when the night is deepest. Look up. If you live in a city, you will not see a black sky but a grey-orangey fog. This is a consequence of the dispersion in the atmosphere of light particles (photons) emitted by our artificial lighting. Indeed, all illuminated surfaces (roads, walls, trees, rocks, etc.) reflect part of the light they receive. Most of these surfaces are called "Lambertian" surfaces, as they scatter light in all directions*. Thanks to this property, our eye is able to see these surfaces.

Light scattering by a Lambertian surface
(Source: https://e-cours.univ-paris1.fr/modules/uved/envcal/html/rayonnement/2-rayonnement-matiere/2-3-reflexion.html)

However, this reflected light doesn't just tickle our retina. It is again reflected by other surfaces, including micro-droplets of water suspended in the air and fine particles generated by human activities. The result is this perpetual luminous halo which envelops large cities over a radius of several kilometers. An ideal setting to paint like Turner? While light pollution prevents amateur and professional astronomers from observing the stars, it also has a serious impact on our health.

Our biological clock is disturbed

Our days are punctuated by different biological processes, including the alternation of being awake / being asleep, which is repeated in a 24-hour cycle. This is called the circadian cycle. This term was composed from the Latin words "circa" (around) and "dies" (day). The circadian cycle is coordinated primarily through the body clock, which originates in the hypothalamus, an area located at the center of human brain. There are two entities called “suprachiasmatic nuclei” made up of tens of thousands of neurons whose electrical activity has a cycle of about 24 hours. In fact, experiments have shown that this cycle can vary from one person to another, from 23 hours and a half to 24 hours and a half. Therefore, there are external factors that make it possible to refine the duration of the circadian cycle in order to fix it on our 24-hour days. The most important of them is none other than light.

At the back of our eye is the retina, a membrane made up of photoreceptors such as cones, which allow color vision, and rods, which allow black and white night vision. These photoreceptors transmit a nervous signal to the visual cortex, through which the brain builds a mental image of our surroundings. But that's not all. Cones and rods are also connected to particular cells: “melanopsin-containing retinal ganglion cells”. They react to slow variations in brightness and are linked to the suprachiasmatic nuclei. Thus, daylight activates the transmission of a nervous signal to our biological clock. Conversely, when it is dark, this signal is interrupted. Thanks to this external stimulus, the biological clock is able to synchronize itself over 24 hours. It is also through that this phenomenon is responsible for our bodies to adapt to jet lag within a few days.

Extract from ANSES report

Effets sur la santé humaine et sur l’environnement (faune et flore) des diodes électroluminescentes (LED)

The light, coming from the left in the diagram, hits the bottom of the retina (on the right). The information emitted by the cones and rods then travels up through the ganglion cells to the brain.

Fine, however… Under the pretext of comfort and safety, all the sources of light that surround us at nightfall disrupt our internal clock in spite of us. If you are fully aware that the glow in your room is due to the lamppost located in front of your window, your biological clock, on the other hand, interprets this phenomenon as proof that it is still daytime. And this is not without consequences for a certain hormone.

The melatonin fairy

For a long time, you were led to believe that you fall asleep thanks to the sandman. In fact, this is the role of the melatonin fairy. This hormone is secreted mainly by the pineal gland, which receives its instructions - from the suprachiasmatic nuclei. From the internal clock! In fact, the secretion of melatonin takes place from evening to early morning and peaks in the middle of the night. This hormone is primarily responsible for the process of falling asleep and, more generally, for regulating sleep. But its role goes far beyond. It is the conductor of most of the biological rhythms during which the other hormones that run our bodies are secreted. Thus, melatonin could have an influence on body temperature, the immune system, appetite and even blood sugar.

But here's the rub. The production of melatonin is inhibited when the retina catches light. In fact, exposure to artificial light sources at night, by disrupting the secretion of melatonin, can cause circadian rhythm disturbances and lead to an increased risk of developing several disorders: sleep, diet, immunity or mood... up to cancer or depression. Nothing very joyful, in short.

The problem of screens

Screens, again and again screens… Unlike other sources of light pollution, we have complete control over these objects. But could we do without them at night? Hardly.

In 2018, a report from ANSES (French Agency for Food, Environmental and Occupational Health & Safety) underlined the potentially harmful effects of light-emitting diodes (or LEDs), which make up screens, on our health. In fact, by their constitution, most LEDs emit a large proportion of blue light. However, the melanopsin ganglion cells in our eye contain. - melanopsin (did you get it?), A photosensitive protein that is particularly receptive to wavelengths around 480 nm: in other words, blue light.

Here is the problem. Screens, which are often consulted before going to sleep (television, cell phone, etc.), provide a dose of artificial light at a time when the eye is supposed to be in the dark. In addition, this light is composed of wavelengths very stimulating for our melanopsin ganglion cells. Farewell, secretion of melatonin!

Today, thanks to their advancements in terms of energy savings and control possibilities, LEDs tend to equip more and more light sources, ranging from car headlights to street lamps, by way of office ceiling lights and household bulbs. A "cool white" interior lighting is not a problem during the day and can even have a beneficial effect on productivity, but it is no longer the case in the evening. It is partly for this reason that the French Decree on light nuisance of December 27, 2018 requires that the color temperature of street lighting (for new installations) not exceed 3000 Kelvin (K). Indeed, the lower the color temperature, the lower the proportion of emitted blue light.

Electromagnetic spectrum of a "cold white" LED (4000 K)

We can clearly see an emission peak in the blue.
(Source : https://leclairage.fr/led/)

Comparison of public lighting in 3000 K and 4000 K
(Source: https://www.cityofelgin.org/2402/LED-Street-Light-Project-Survey)

What can I do ?

Unless you live far from any civilization, light pollution, whether it comes from outside or inside your home, inevitably has an impact on your nights. In general, it is recommended to have a marked light contrast between day and night. In other words, you must be exposed to natural light as much as possible during the day in order to counterbalance the exposure to a parasitic light in the evening. Regarding blue light, ANSES recommends in particular the use of "warm white" bulbs, whose color temperature is equal to 3000 K (or less), for domestic lighting. Finally at night, it is better to sleep in complete darkness.

Light pollution has consequences not only on human health, but also on biodiversity. This aspect will be the subject of a second Cortex article in several weeks.

Anyway, the best solution against light pollution remains… to turn off the light.

*For more information on the scattering of light: read the article written by Virgile Guei (https://www.le-cortex.com/en/article/how-particles-interfere-with-light)

References :

1. Anses, Effets sur la santé humaine et sur l’environnement (faune et flore) des diodes électroluminescentes (LED), april 2019 (https://www.anses.fr/fr/system/files/AP2014SA0253Ra.pdf)

2. Arrêté du 27 décembre 2018 relatif à la prévention, à la réduction et à la limitation des nuisances lumineuses, version in force on September 28, 2021

3. https://www.ecologie.gouv.fr/arrete-du-27-decembre-2018-relatif-prevention-reduction-et-limitation-des-nuisances-lumineuses

4. https://www.inserm.fr/dossier/chronobiologie/

5. https://lecerveau.mcgill.ca/flash/i/i_11/i_11_cl/i_11_cl_hor/i_11_cl_hor.html

6. https://www.unistra.fr/uploads/media/CP_Melanopsine_23-06-09.pdf

7. https://www.anses.fr/fr/content/led-les-recommandations-de-l%E2%80%99anses-pour-limiter-l%E2%80%99exposition-%C3%A0-la-lumi%C3%A8re-bleue

Tags article : "Biological clock"

Light pollution, we are all disturbed