Despite what some may think, the earth rotates around the sun, and on itself. This rotation on itself causes a day/night alternation to which all species on the planet have adapted.
Example with the eyes
The function of the eyes is to transform ambient light into an electrical signal, transported by the optic nerve and perceived by the brain, which interprets the signal: this is called vision.
Biological evolution has done its work: during the day, a period of activity, the body's organs consume energy to fulfil their function: the muscles, the stomach, and so on.
At night, the body needs to rest to regenerate some of its functions.
To perform this task, the cones at the back of the eye use pigments (the opsins) that degrade as they are used: they transform the light received into an electrical signal.
At night, when the eyes are at rest, the body regenerates these pigments so that they can be used again the following day.
Take away the darkness, and you take away the rest of the eyes, which means pigment regeneration and, ultimately, impaired vision.
Note that these biological phenomena are not instantaneous: the effects take several years to be felt.
This Circadian rhythm is necessary for the whole human body.
The brain communicates with the body's various organs. It uses the nervous system (electrical communication) to give commands to a muscle, for example. But it also uses another means of communication, a chemical one: hormones.
Hormones are used for a whole range of functions: regulating temperature, accelerating the heartbeat, signalling satiety. These chemical messengers are (among others):
* serotonie (thermal regulation, pain control, mood)
* adrenaline (stress, heart frequency, pupil dilatation response)
* dopamine (pleasure, motivation)
* endorphins (physical activity, pain, orgasm)
But the one that interests us for the cricadian ryhtm is the melatonine.
Often referred to as the sleep hormone, melatonin could easily play the role of the sandman in a molecular remake of "Bonne nuit les petits": in the evening, melatonin helps us fall asleep.
Melatonin production increases at the end of the day, when light levels drop. It's the signal that it's almost time for bed! Its concentration in the blood rises to a peak around 3 or 4 a.m., then falls back to a minimum by the time it's time to get up.
But outside light can change all that: the light perceived by our retina is directly transmitted to our main internal clock, located in a part of the brain called the hypothalamus and then to the "pineal gland") responsible for secreting melatonin.
So, when we expose ourselves to too much light in the evening, this system sends the following message: "Don't bother starting to produce melatonin: it's still daylight, it's not time to go to sleep".
And that's how we become unable to fall asleep after a whole evening spent in front of the powerful light of a TV, computer or telephone screen.
In the long term, the person responsible for disrupting chemical messages is called an endocrine disruptor.
The definition of endocrine disruptors is still the subject of international debate. However, the definition proposed by the World Health Organization in 2002 is the most widely accepted.
"An endocrine disruptor is a substance or mixture that alters the functions of the endocrine system, thereby inducing adverse effects in an intact organism, in its offspring or in (sub)populations.
Source : French National Institute for Research and Safety
The "star" compounds of known endocrine disruptors are :
As we saw with the sleep hormone, light influences the production (or otherwise) of melatonin.
Light is not a chemical compound, but the effects are the same, since it disrupts the hormonal system.
Effects include
Plastics, pesticides and other chemicals are well and truly "carcinogens". Shouldn't we add light?
A study conducted in China in March 2024 is made on the link between artificial light and stroke.
It's time to wake up to the problem.
BACKGROUND
We set out to explore the associations between nocturnal artificial light (LAN) and air pollution with the risk of cerebrovascular disease (CeVD).
METHODS
We included a total of 28,302 participants enrolled in Ningbo, China, from 2015 to 2018.
Outdoor lighting network and air pollution were assessed by satellite and land-use imagery.
Stroke cases were confirmed by medical records and death certificates and subdivided into ischemic and hemorrhagic stroke.
Cox proportional hazards models were used to estimate 95% hazard ratios and incident strokes.
RESULTS
A total of 1,278 cases of cerebrovascular disease (including 777 cases of ischemic stroke and 133 cases of hemorrhagic stroke) were recorded over 127,877 person-years of follow-up.
In single exposure models, the risk ratios for cerebrovascular disease were 1.17 (95% CI 1.06-1.29) for outdoor LAN, 1.25 (1.12-1, 39) for particles with aerodynamic diameter ≤2.5 μm, 1.14 (1.06-1.22) for particles with aerodynamic diameter ≤10 μm and 1.21 (1.06-1.38) for NO2 for each increase in the interquartile range.
The results were similar for ischemic stroke, while no association was observed for hemorrhagic stroke.
In multiple exposure models, associations between LANs and outdoor PM and cardiovascular disease persisted, but not for ischemic stroke.
Furthermore, no interaction was observed between the outdoor lighting network and air pollution.
CONCLUSIONS
Levels of exposure to outdoor lighting and air pollution are positively associated with the risk of cardiovascular disease.
Furthermore, the adverse effects of local outdoor lighting and air pollution may be mutually independent.
Outdoor Light at Night, Air Pollution, and Risk of Cerebrovascular Disease: A Cohort Study in China (pdf).
In the study published this 20th of september by the Frontiers in Neuroscience journal, scientists say exposure to artificial outdoor light at night was correlated with a higher risk of developing Alzheimer's disease—though the study didn’t make a direct causal link.
Researchers used satellite acquired data to determine nighttime light intensity in states and compared that data with Alzheimer's disease prevalence in the same states, finding that a positive correlation was found in states with the most light pollution in every year studied (from 2012 to 2018).
States with the most light pollution—and the highest Alzheimer's disease prevalence—included Florida, Connecticut and New Jersey, while Montana, New Mexico, Vermont, Idaho, Wyoming and Oregon had lower instances of both.
Nighttime light pollution had a stronger correlation to Alzheimer's disease risk than alcohol abuse, chronic kidney disease, depression, heart failure and obesity, the study showed, but less of a link to the conditions of atrial fibrillation, diabetes, hyperlipidemia, hypertension and stroke.
In people under age 65, however, nighttime light exposure was more strongly associated with Alzheimer’s prevalence than any of the other disease factors studied.
Light pollution is the latest in a list of environmental factors that studies suggest could be linked to the risk of developing Alzheimer's disease, including living near green spaces, long-term exposure to air pollution and chemical exposures.
