Hong Kong Island at night. The city may be the most light-polluted anywhere. Photo: Martin Chan

How Hong Kong's urban light pollution creates 'mini jet lag'

Bright nights make our bodies think it's daytime, with brain and heart working faster - disruption that is associated with higher rates of cancer


Astronomer Dr Jason Pun Chun-shing of the University of Hong Kong's department of physics has been studying light pollution for nearly a decade. He says people often ask him if he is crazy. Hong Kong is supposed to be bright, they say. Why are you even talking about light being some kind of pollution?

The is famous for its nightscape: neon signs advertising market stalls, pawn shops and restaurants; illuminated skyscrapers; swanky malls that stay open and stay lit well into the night. "When I walk at night around some of these commercial centres, it is so bright you almost want to wear your sunglasses," Pun says.

In our collective imaginations, cities are meant to be bright. But as studies begin to show that too much light can be detrimental to health, and fewer of us are able to see the stars when we look up, are cities getting too bright for our own good?

Hong Kong isn't alone in celebrating light. Paris is still known as the City of Light; the English resort of Blackpool relies for tourism on its annual illuminations, when more than a million bulbs light a distance of 10km.

This celebration of artificial lighting is perhaps unsurprising, given how recently electric streetlights became the norm. It is easy to forget that being bathed in light is a relatively modern phenomenon. Although electric street lights first began appearing in European capitals in the mid-1800s, widespread street lighting did not become common until well into the 20th century.

In the years since, our cities have come to be lit in the extreme. Where once lighting was a luxury, now, a clear view of the night sky is what is become rare. With these metropolitan centres continuing to expand, things are going to continue to get worse n the night sky is going to continue to get brighter and our chances of seeing the stars anywhere other than in remote rural areas is going to get slimmer. That is why Pun decided

Hong Kong is often touted as the most light-polluted city in the world - a view supported by a recent study from Pun and his department, Hong Kong Night Sky Brightness Monitoring Network (NSN), which measured so-called night sky brightness.

"We set up about 18 stations around the city, in all sorts of living environments, from the commercial urban centre, to more residential neighbourhoods, to relatively rural areas," he explains.

Then they compared the levels of light to the standard provided by the International Astronomical Union, which states how bright the sky would be without artificial light. In the most-lit areas, it was 1,000 times brighter.

"Similar studies in major capitals like Berlin and Vienna," says Pun, "would find something more of the order of 100 to 200 times brighter."

But with light pollution studies still in their infancy, and without any strict international standards on how to quantify the extent of light pollution, it is hard to say for sure whether Hong Kong is the most light-polluted city. Other candidates that are often cited by those with the best view - astronauts - include Las Vegas, Tokyo, Seoul and New York.

And Hong Kong, like many cities around the world, is proud of its illuminated city. "The brighter the better," Pun explains, mimicking a chirpy toothpaste ad. "Brighter means more prosperous. We have a nickname for Hong Kong: the Pearl of the Orient. So I suppose a lot of people take this actually as a badge of pride without rethinking what all this brightness means."

That can include health problems. "There is a cascade of changes to our physiology that are associated with light exposure at night," says Steven Lockley, a neuroscientist and an associate professor of medicine at Harvard Medical School. He has looked at the impact of light on human physiology, including on alertness, sleep, and melatonin levels.

Because humans evolved in a 24-hour light/dark cycle known as the circadian clock , any light after dusk is "unnatural", Lockley says. "When we are exposed to light after dusk, our daytime physiology is triggered and our brains become more alert, our heart rates go up, as does our temperature, and production of the hormone melatonin is suppressed."

Has the way city dwellers live, removed from natural light patterns, confused our bodies? "Not so much confused as shifted: we've been shifted later," Lockley says. "What happens when people go camping? If you don't have sources of electric light, then you go to bed earlier, shortly after the sun is gone down, and you sleep for longer."

Every day we don't go to bed at dusk, we experience what Lockley calls "mini jet lag".

His colleague, Ken Wright at the University of Colorado in Boulder, in the United States, conducted an experiment on camping. Wright found that for campers, midnight was the middle of the night: living in brightly lit cities has artificially lengthened our days. "We go to bed later, we don't sleep as long, and we don't know of the long-term health impact of changing," he says.

There have been studies about how changes in circadian rhythms - which may be explained by exposure to light at night - can have an impact on humans. Studies of shift workers found that circadian disruption is probably carcinogenic to humans; female night workers, for instance, were found to have a higher risk of breast cancer than women who do not work at night.

"As a society we need to think, do we really need some of these amenities that are putting light pollution into the environment?" Lockley says. "Do we need 24/7 garages, do we need 24/7 supermarkets, do we need 24/7 TV? [Until] 1997 the BBC turned off and there was the national anthem and we all went to bed."

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This article appeared in the South China Morning Post print edition as: Urban light pollution creates 'mini jet lag'