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 santéhumaine et sur l’environnement (faune et flore) des diodesélectroluminescentes (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
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