Light is a beautiful thing, which we’re particularly aware of when the first rays of spring sunshine helps lift our mood after a bleak, dark winter. Light is also a luxury, something that with the flick of a switch we can call up independently of Mother Nature. It’s a symbol of security and wealth.

Yet too much light is bad for us. Artificial light from street lights, lit billboards, store windows, even the light shining out of people’s homes at times when it would naturally be dark, has negative effects both for humans and the environment.

"We have to come to terms with the fact that more and more light doesn’t only have positive effects, like increased security or improved conditions for economic production -- it also has negative effects, for example on ecology," says Franz Hölker, an ecologist with the Berlin-based Institute for Freshwater Ecology and Inland Fisheries and head of the Leibniz Research Alliance “Loss of the Night” project.

For that reason, scientists refer to the overabundance of artificial light as “light pollution.” And that flood of manmade light is increasing by an estimated 5% to 7% every year.

According to information from the International Dark Sky Association, 99% of the population of Europe and the United States live under light-polluted skies. One result is that even on clear, new-moon nights we can hardly see the stars in the sky anymore.

Source: NASA

In cities, some 200 to 500 stars can now be seen with the naked eye: it used to be 2,500. Half of Europe can no longer see the Milky Way. Astronomers are thus building their observatories far away from densely built-up areas – in the Chilean Atacama desert, for example, or on the Canary Islands.

But beyond frustrations for astronomers, the overlit nights have much broader effects than that: they disturb eco-systems, and interfere with the chrono-biological rhythms of animals and humans alike. Clocking what’s day and what’s night is, after all, nature’s job.

Artificial light at night disturbs the growth cycles of plants, makes it increasingly more difficult for migratory birds to find their way – and schools of fish perceive brightly lit bridges as insurmountable barriers.

Suddenly species that are normally active either during the day or at night are getting in each other’s way – bats and birds, for example, both out on the hunt for food.

The effects of artificial light are particularly pernicious for insects. Hundreds of billions of them spend day after day buzzing around street lights and then die, burnt to a crisp or from exhaustion.

"Metal halide and mercury vapor lamps might as well be called ‘insect killers,’" says Gerhard Eisenbeis, a zoology professor at the University of Mainz. "Outdoor artificial lighting is and remains ecologically sensitive."

But people too suffer from the permanent glaring lights in streets, in and on buildings, in store windows. These artificial lights disturb hormone balance and hence our inner clocks – and that leads among things to sleep difficulties.

Source: NASA

"Light at night postpones release of the sleep hormone melatonin and hence decreases the amount of time for quality sleep," says Dieter Kunz, head doctor in the sleep medicine department at Berlin’s St. Hedwig hospital. That means that it’s harder to get to sleep, there’s overall less sleep time – and it’s harder to wake up.

Sleep plays a major role in various learning processes, memory formation, health of the immune system and a good deal more. "Healthy sleep is of overwhelming importance for the functioning of body and brain," says Kunz. Chronic lack of sleep is in partly responsible for the prevalence of such problems as high blood pressure, diabetes and obesity.

Much more than just pyschological

Individual experiments have produced results to the effect that increased amounts of light at night could also play a role in the earlier onset of puberty.

Exposure to light during sleep harms the organism, researchers at Johns Hopkins University in Baltimore found as a result of experiments with diurnal mice. The experiments showed that even tiny amounts of light at night led to changes in the brains of the rodents that resembled changes in the brains of depressive (human) patients.

Another American research team at Ohio State University in Columbus reached the same conclusion in experiments with Siberian dwarf hamsters. After four weeks of sleeping for eight hours in twilight conditions they started to exhibit symptoms of depression, the scientists report in an article in Molecular Psychiatry magazine.

As soon as the hamsters were able to sleep in complete darkness again, their joy of life returned. As the day-night cycle of diurnal animals is similar to that of human beings, the study results can be applied to humans.

According to a report on the effects of artificial light on health published by the European Commission, sleep disturbances and depression are not the only consequences of artificial light: there could also be a relation to higher levels of breast cancer. The reason for that is that estrogen levels rise when less melatonin is formed – and too much estrogen is considered a risk factor for breast cancer.

Israeli researchers reached similar conclusions in a study published five years ago: the risk of getting certain types of cancer such as breast cancer and prostate cancer was higher in strongly light-polluted areas.

The Israeli scientists also assume the reason is hormone imbalance: when light gets through eyelids to the retina at night, the production of melatonin is inhibited – and it is precisely that hormone that makes our bodies more resistant to certain types of cancer.

Shift workers suffer particularly from the phenomenon. "When people work at night, their sleep-wake cycle is put off by eight or nine hours, and the various physiological rhythms follow the delay to differing extents and at different speeds," says Dr. Barbara Griefahn, a specialist in occupational health at the Research Centre for Working Environment and Human Factors at Dortmund’s Technical University.

The result: female medical staffers and workers on night shift have a markedly higher rate of breast cancer than the rest of the female population. Depression, cardiovascular diseases, diabetes and a tendency to gain weight are also markedly more present in night shift workers. Put another way, too much light can make you sick.