What is light pollution?

Light pollution sums up all adverse effects of artificial light at night. This includes the loss of our view of the stars, the detrimental effects on our health, the threat to wildlife and ecosystems, the impact on safety, the aesthetical damage to our nightscapes, and the waste of energy. Light pollution, especially in the ecological context, arises from every form of artificial light, even if it is well planned and necessary. However, currently a vast amount of light is wasted by bad lighting design or simply because light is produced without anybody using or even noticing it.

Light has a very positive connotation. It is usually seen as a sign for wealth, modernity, and safety. Modern cities are often portrayed by their illuminated skyline. Worldwide, the use of artificial light is increasing in area and intensity by up to 10% each year.

This increase has been fuelled by the introduction of energy efficient LEDs, which results in a rebound effect: instead of using the same amount of light and saving energy, more light is used, even if it is not necessary. Light pollution can have very direct, easily recognisable effects like glare in humans or attraction of insects. However, many effects are subtle, including health effects, the slow change of entire ecosystems, or the loss of the beauty of nightscapes and the growing aversion against natural nights.

Starry skies: The loss of the oldest world heritage

Astronomers were the first to detect the effects of light pollution when weaker stars disappeared from view. Today, more than 99% of Europeans live under an artificially illuminated sky, that means they can’t see the full extent of our night sky, which is recognised by the UNESCO as the oldest world heritage. Given the excitements of modern life, this might not seem a great loss, but the stars have been important for humankind since we can remember. For millennia, they have been the fundament of navigation and time-keeping, alerting farmers and hunter-gatherers to seasonal events. They have also been the centre of ancient myths. Even today, many indigenous cultures connect to their heritage through the stars. References to the night sky can be found throughout Western literature, music and art.

The connection to the stars is still important for modern societies. Worldwide, stars hold fascination to people, connecting us beyond borders, skin colour, social status, or religion. We all share one sky, and for many people the view into its beauty brings hope and a sense of belonging, especially during times of crisis like war or the recent Covid pandemic.

In theory, stars are everywhere, and you need no expensive equipment to enjoy the beauty of a night sky. For many children, looking at the stars is the first contact with natural sciences and technology. Different to physics or biology, it captivates boys and girls to the same extent. This first contact is often the beginning of a career in one of the STEM-fields, an area we need more experts to ensure high quality of life. But even if you won’t become a nuclear scientist after gazing into a starry sky, it is a breathtaking experience every human should have access to.

Night life: All organisms need darkness

Half of our world’s time is nighttime. The day-night-rhythm is so basic that all organisms have developed circadian rhythms that control activity, physiology, reproduction and even gene expression. Many organisms are adapted to the low nocturnal light levels. Artificial light blinds them, creates areas of fear and increases habitat fragmentation. Small mammals like mice, hedgehogs, and bats avoid illuminated areas or reduce their activity times.

Other nocturnal animals are drawn to the light. Migratory birds follow the light for kilometres. For millions of birds, this flight ends in a deadly collision with an illuminated building. With insects, it’s more complicated. Most people are aware that insects are drawn towards the light, but there is evidence that many flying insects reduce activity, including food intake and mating, is actually reduced by light at night. No matter if an insect is caught at a light source or waiting for darkness to fall, these insects will not feed, not reproduce, nor will they pollinate the plants other animals including us humans need as food.

Diurnal animals on the contrary can extend their activity times which might give them advantages over the nocturnal species. However, this lack of resting time and sleep  comes with a price: In birds, stress levels are increased, and the immune system is weakened.

The reason is that light controls our body. All organisms, even bacteria and fungi, have internal clocks that create endogenous rhythms. They control sleep, brain activity, hormone production, reproduction, immune system, and gene activity. Artificial light interrupts these rhythms and interferes with an organism’s functionality on the most basic level.

Plants, our primary producers of biomass and thus food, use light for photosynthesis, no matter at what time. But they need darkness at night for regeneration, and they need seasonal changes to control their reproduction. Artificial light at night disrupts this by reducing seed production. Depending on the species, leaves become harder for insects to feed on, or the leaves die early due to light-induced cell-destruction.

By impacting individuals and even interfering with plan growth, light changes whole ecosystems. Light sensitive species decline, light tolerant species might replace them. Biodiversity is reduced, food webs are interrupted. Nocturnal habitats are fragmented by bright lines that connect towns. The light of settlements creeps into natural areas. Many organisms are so light sensitive that even the skyglow of a city 100 kilometre away affects them.

Light pollution isn’t an isolated problem. It reduces the resilience of an organism, making it more vulnerable to climate change, habitat loss, other pollutants, alien invading species, or over-exploitation of natural resources. Light pollution adds to a growing number of environmental problems, increasing their effects in a way we can’t predict. So far, we have ignored the importance of light pollution because as a diurnal species we don’t realise the importance of natural darkness for all living beings. It’s high time to change this and think nature conservation 24/7 instead of only during the day.

But light is good for us humans, isn’t it?

There is a growing amount of evidence that artificial light at night is harmful for our health and well-being. Studies have found links between the amount of light in a region with sleeping disorders, use of sleep medication, depression, suicide, dementia, obesity, type-2 diabetes, cardiovascular diseases, complications at birth and cancer, (i.e. breast, prostate, thyroid). Unclear is how strong the influence on our health is, if light is the cause of these health issue, and which pathways lead to the health problems.

Under natural conditions our brain produces melatonin at night, a hormone that controls almost all bodily functions, including sleep. Artificial light can suppress the melatonin production, which if it’s happening over a long period of time interrupts our internal rhythms and increases the risks of all the health issues linked to light pollution.

Light also can be a direct disturbance. If unwanted light shines into private spaces like gardens and homes, it’s annoying. Using blinds might seem a solution, but it takes away our free choice to participate in the night, enjoy relaxing light, or gaze into the stars. For many people, sleeping behind blinds to keep the artificial light out is stressful. Stress, especially over a long period of time, affects our health and can result in mental problems, cardiovascular diseases, and type-2 diabetes. Stress also impairs our immune system by causing chronic inflammation, and stress plays a role in many forms of cancer. We are aware of the consequences of stress caused by noise, but when it comes to light we ignore the same processes in our body.

Low melatonin production and stress both cause sleep problems, one of the most common problem in modern societies. The reasons for sleep problems are divers, and so are its consequences. Lack of sleep not only reduces our ability to concentrate and read emotions, it also weakens our immune system and cancer defences, and it increases our risks for all the other health issues related to light pollution. It seems that even if your body does produce normal amounts of melatonin, lying awake because the unwanted light outside bothers you too much is bad enough for your health.

Last but not least, light pollution might interfere with our health in a more subtle way. In the last years there has been research on the benefit f the night sky on our health. It turns out that people who feel a connection to the night sky and who have the chance to look at it, range higher on the mental health score. Stargazing might be a valuable anti-depressant.