The Church of the Good Shepherd at Tekapo is seen with the Milky Way as a backdrop. Tekapo's curbs on light pollution has made it a magnet for tourists and stargazers wanting a spectacular view of the night sky.

OPINION: As Christchurch rebuilds, there is an urgent need for local and national politicians and city planners to take heed of lighting.

There is now a growing world-wide realisation that too much city lighting is bad – it's bad for the environment, bad for human health, bad for the economy and bad for tourism, including the strongly growing astro-tourism sector.

The Mackenzie District at Tekapo is a flagship case of how street and security lighting should be installed and how to limit illuminated advertising signs. A lighting ordinance was incorporated into the Mackenzie District Plan in 1981 and is now part of the Resource Management Act. Christchurch politicians should take note of this, and try to follow suit in the city. Opportunities to rebuild a large part of any city come rarely enough, and lighting needs to be a key part of any city plan.

The key features of the Mackenzie lighting ordinance are first, that lights should be shielded so they emit no light horizontally or upwards, but only down to the ground where the light is needed. Such lights are called full cut-off luminaires.

Secondly we should bear in mind that most white lights (including sunlight) actually emit a mixture of all the colours of the rainbow. The lighting ordinance limits the emission of the blue and violet components of light, as these scatter in the atmosphere the most readily and brighten the night sky, thus hindering star-gazers and researching astronomers. Visitors to Tekapo will notice the predominance of low-pressure sodium lights, with their strong orange glow and the complete absence of any blue light.

Thirdly, the ordinance precludes flood lighting and sports lighting after 11pm, and discourages illuminated advertising signs.

Why is exterior lighting so important? There are four broad reasons, though each has many inter-related tentacles. These reasons are to protect the environment, to safeguard human health, to save electricity and to promote the growing market for astro-tourism by keeping the night sky dark.

The environment has been severely damaged in and around all large cities where street and other lights are illuminated indiscriminately all night long. Many species of animals and insects are nocturnal, and artificial lighting has severely damaged their habitats. Also, birds are disorientated when office buildings are illuminated at night, causing birds to fly into lit windows and be stunned. Moreover all species have evolved over millennia to adapt to a diurnal cycle of alternating light and dark. Artificial light that produces 24-hour illumination affects many species, including trees and plants. Erratic behaviour of animals is often the result, including disturbance to mating cycles, extra predation and psychological stress.

There is a common belief that bright street lights at night are necessary for urban safety. Some lighting does promote safety, but too much causes glare and light trespass. Glare blinds our vision and reduces the visibility of objects and people in the direction of the glare. Light trespass is the spill-over of light to places where it is not needed, including into bedroom windows. Both are highly undesirable.

In recent years there has been a growing body of evidence that points to the harm being done by artificial light at night on human health. The retina of our eyes has cells called retinal ganglion cells, which are especially sensitive to blue light. These are not used for imaging (unlike the rods and cones in the retina) but send a signal to the hypothalamus, deep inside our brain, which in turn controls the hormone production in glands such as the pituitary, pineal, adrenal and thyroid glands.

Some of these glands produce hormones after we are exposed to blue sunlight in the mornings after rising from bed. These include cortisol (stress response), serotonin (impulse control), dopamine (pleasure, alertness) and others which control hunger feelings, sexual libido and body metabolism.

Even more important are hormones produced only in the absence of blue light at night as we sleep, including melatonin, and hormones which control blood pressure and body growth.

Melatonin is almost completely suppressed by tiny amounts of stray blue light at night, such as from electronic devices (cell phones, TV screens, digital clocks), even if our eyes are shut. If melatonin is not produced, our day-night circadian rhythm is disrupted, and we feel stressed and tired. Jetlag is also a consequence of disrupted melatonin production, and most of us have experienced that after long flights to or from New Zealand.

Melatonin also helps with our auto-immune response to destroy viruses and control cancer as it activates cells called N-Killer and cytotoxic T cells which help fight some autoimmune diseases (asthma, arthritis) and also some types of cancer (notably breast and prostate cancers). There is now clear evidence that night-shift workers have higher incidence of these cancers as a result of disrupted hormone production, especially of melatonin.

The message is clear. It is important that we sleep in total darkness, and especially blue light at night even in tiny quantities is dangerous for human health. Light trespass from blue-emitting street lights through thin bedroom curtains is particularly harmful.

The modern industrialised city-dweller throughout the western world has seen soaring cancer rates in the 20th century. Cancer has a multiplicity of causes, but pervasive city lighting in the big cities must take at least a small part of the blame. Just how much remains to be seen – and more research is needed. But it is striking that cancer rates show large regional differences between developed and undeveloped countries.

The third major reason why it is important to control street lighting is that every photon of light that goes upwards into the air represents wasted electricity to produce that light. Anyone who has flown over a major city at night will have seen that it is awash with light going up into space.

The view of the Earth is even more dramatic from space. Astronauts in the Space Station have photographed the entire Earth's surface at night. North America (east and west coasts), all of Western Europe, all of Japan and all the large population centres of China are completely ablaze with light. And the residents in those cities, when they look up, see practically no stars. In North America the annual cost of electricity for street lighting is around $4 billion. It is estimated that if street lights only shone down with full cut-off light fixtures, then about a quarter of the cost could be saved, or around a billion dollars a year. Worldwide, the figure would be much higher. This is a clear win-win situation.

Local authorities responsible for street lighting could save huge amounts of money and at the same time, protect the environment, deliver healthier lives for city-dwellers and allow us to see the stars.

The fourth reason for controlling street and other external lighting is just that – to allow us to see the stars. That was the original motivation for the Mackenzie lighting ordinance in 1981, which was one of the first in the world at the time. It was drafted explicitly to protect the night sky above Mt John Observatory at Lake Tekapo. More recently it has been a boon for a flourishing astro-tourism industry in the Mackenzie.

Today over 100,000 tourists come to the central South Island each year explicitly to see the pristine unpolluted night sky and to enjoy the amazing view of the stars. It is a sight many from Asia have never seen before, and it is breath-taking.

Stargazing may be less important for Christchurch city, which will never be as dark as the Mackenzie. But it is not unimportant. Amateur astronomers and stargazers still can gaze upwards from Christchurch and see the beauty of our southern skies. It is an asset also worth protecting for that reason alone.

In 2007 a conference was held in the Canary Islands of Spain on light pollution and starlight. It was attended by astronomers, lighting engineers and environmentalists around the world, including Graeme Murray of Earth and Sky at Tekapo. The Starlight Conference was sponsored by Unesco, and Unesco's Starlight Declaration was issued as a result (see www.stralight2007.net). Parts of the Declaration are worth quoting.

"It is declared that: An unpolluted night sky that allows the enjoyment and contemplation of the firmament should be considered an inalienable right equivalent to all other socio-cultural and environmental rights. Hence the progressive degradation of the night sky must be regarded as a fundamental loss…

"Control of obtrusive light must be a basic element of nature conservation policies since their impact on several species, habitats, ecosystems, and landscapes.

"Protection of the astronomical quality of areas suitable for the scientific observation of the Universe must be given priority in national and international scientific and environmental policies.

"The intelligent use of artificial lighting that minimises sky glow and avoids obtrusive visual impact on both humans and wildlife should be promoted. This strategy would involve a more efficient use of energy so as to meet the wider commitments made on climate change, and for the protection of the environment.

"Tourism, among other players, can become a major instrument for a new alliance in defence of the quality of the nocturnal skyscape. Responsible tourism, in its many forms, can and should take on board the night sky as a resource to protect and value in all destinations."

It is not only street lights which need controlling so as to shine only downwards and have no blue component, which scatters far more in the atmosphere than yellow or red light.

The city council should also disallow all flood-lighting, ensure that security lights have motion sensors so they are off most of the time, and internally illuminated advertising signs should be completely outlawed in Christchurch.

I was shocked when driving along Victoria St at night recently to see that so many of the new buildings have huge illuminated signs announcing their corporate owners to the world. Such signs are bad in every way – for the environment, for human health, for the economy and for creating light pollution and light trespass that causes sky brightness and greatly impedes stargazing.

In Europe, France led the way when, in 2012, the French minister for energy and environment unveiled a proposal for lights in and outside shops, offices, and public buildings – including the flagship Louis Vuitton store and the Lido cabaret house on Paris' Avenue des Champs Elysees – to be turned off between 1am and 7 am, starting in July 2013. The plan, to be applied across French cities, towns and villages, is aimed at saving energy and money and showing "sobriety", Minister Delphine Batho said.

In Croatia they also have implemented an "enlightened" approach to city lighting, with controls in place for major cities such as Zagreb and Dubrovnik. Dubrovnik inaugurated its lighting master plan in 2009, with the aim of reducing lighting costs by 60 per cent and minimising light pollution.

Any discussion of the harmful effects of light pollution needs to discuss the latest technology in lighting, the light-emitting diode, or LED. Lighting engineers love this new solid-state light source, given that they use only a fraction of the electricity of many traditional light sources for the same amount of light, and they have a much longer life (at least 10 years and perhaps up to 50 years), thereby dramatically reducing maintenance costs too.

So it seems that LEDs are everyone's answer for efficient lighting of our streets.

But there is a worrying and insidious aspect of LEDs which urgently needs to be understood by all concerned. The first LEDs were installed in street lights a decade ago, and they had a large blue component in the light emitted, giving a harsh white light as perceived by the human eye. This strong blue component of many LEDs is disastrous for the environment, for human health and for stargazers. It is important to stop their installation in Christchurch.

Fortunately there is a solution. LEDs can now be coated with a phosphor that converts the blue light to yellow, thereby giving a "warmer" yellower light source with a much reduced blue component. They are still not as good as low pressure sodium lights for limiting light pollution, but they are more economical to operate.

Only these LEDs should be used in Christchurch.

John Hearnshaw is Emeritus Professor of Astronomy at the University of Canterbury. He is also President of the Royal Astronomical Society of New Zealand and Chair of the Aoraki Mackenzie International Dark Sky Reserve.