Living organisms have evolved and adapted to an environment of alternating light and darkness.  It is therefore not surprising that light pollution, which literally results in the disappearance of night, can have negative effects on animals and plants as well as on humans.

The long-term consequences of permanent illumination are still not well-known but most studies come to the same conclusion: dark nights are essential for healthy life.

• Sleep should occur in the deepest darkness possible because melatonin, a hormone that stimulates the immune system, is only secreted in the absence of light. A reduction in melatonin production has been linked to diagnosis and growth of certain types of cancer in night-shift workers. An article published by IDA explores this topic in more detail.
• Melatonin also controls our daily biological rhythms. That is why a decrease in melatonin production leads to sleep disorders.
• Artificial lighting weakens the capacity of the human eye to adapt to darkness. This situation is so noticeable for city dwellers when they take a country walk in the dark that it can cause alarm.

The effects of over-illumination on other organisms are poorly understood. Biologists have, however, come to a consensus on the following points:

• Artificial lighting disrupts biological rhythms by resetting the hormonal processes that control the internal clocks of plants and animals.
• Lucifugous species, which are generally those that are nocturnal or live beneath the ground, bark, etc., attempt to escape from light even at night.
• Over-illumination in cities and along the coast causes migrating birds to become disoriented. Large structures, such as bridges and skyscrapers, then kill birds by the millions.
• Most nocturnal animals are disturbed by artificial lighting and sometimes disappear completely from suitable but illuminated habitats.
• Many insects are attracted to light, which results in huge numbers being killed by unprotected bulbs and predators that take advantage of light sources to locate prey. Both factors bring about a disruption in the food chain.

For more information, consult the article entitled Ecological Light Pollution, which is a literature review on the subject and was published by the Ecological Society of America, or the book entitled "Ecology of the Night".