THE human race evolved in a world of bright days and pitch dark nights. When the sun went down only the moon and the stars provided faint illumination and even the campfires, candles and oil lamps that came later were dim compared to modern lighting.
Our senses have spent the last century trying to come to terms with a new cycle. Whether it's the strip lighting in a late-night supermarket, fl ickering TV and computer screens or the street lamps outside our bedroom windows, electricity has all but eliminated true darkness from our nights.
According to research, this light pollution is affecting our health. Some experts believe that rising rates of certain cancers, obesity, depression, gastrointestinal problems and diabetes could all be explained in part by the disappearance of truly dark nights.
One theory is that darkness is required for the body to produce the hormone melatonin. When its secretion drops the level of oestrogen tends to rise, which has been linked to breast cancer.
A recent study by researchers at Ohio State University found that too much light at night could be linked to depression. In tests mice that were exposed to light for 24 hours showed many more depressive symptoms than those who could escape into a darkened tube.
"The increasing rate of depressive disorders in humans corresponds with the increasing use of light at night, " says Professor Randy Nelson, one of the researchers.
The best evidence of the health benefits of total darkness comes from night-shift workers, who often go without it altogether.
Within weeks of starting night shifts at a TV company, journalist Carrie Dunn, 29, noticed a marked decline in her health. "I started putting on weight, felt sick and thoroughly miserable, " says Carrie, who suffers from Polycystic Ovary Syndrome (PCOS), a hormonal disorder that can lead to weight gain, irregular periods, acne and depression.
Her GP spotted the link between the worsening symptoms of her PCOS and her night shifts. "My unreliable metabolism was completely confused, " she says.
"My body didn't know when it was day, when it was night and my interrupted sleep patterns were making me feel even worse." When she gave up night shifts her symptoms disappeared within a month.
Carrie's experience is far from unique. More than a fi fth of us now work at least some of the time outside the 7am-7pm day.
"Long-term shift workers have a higher incidence of cardiovascular disease and gastrointestinal problems, " says Professor George Brainard, director of the light research programme at Philadelphia's Thomas Jefferson University.
"There's also a potential link to breast and colon cancer in women who are exposed to light at night. In 2007, the World Health Organisation identified long-term shift work as a Class 2 risk for breast cancer."
In March the Danish Government began paying compensation to women who developed breast cancer after long spells of working nights.
Other potential risks are low birth weight babies and longer pregnancies.
However even those of us who don't work graveyard shifts are being affected. Some scientists believe that dramatic increases in obesity and diabetes have mirrored the huge increase in the use of artificial light in recent times.
Why this happens is not fully understood but what is certain is that getting too little darkness upsets the body's internal timekeeper, the circadian system.
"The most important environmental stimulus for keeping this system working is exposure to a regular daily cycle of light and darkness, " says Professor Brainard.
"Darkness at night is a key part of regular biological timing and health."
The circadian system helps to regulate a whole range of our body's core functions, from temperature control to blood pressure. Disrupting it with light at night means disturbing a host of biological activities.
RESEARCHERS are discovering that it takes less artificial light to disrupt our body clocks than was once thought.
Professor Derk-Jan Dijk, director of the Sleep Research Centre at Surrey University says: "Even normal room lights can effect our physiologies."
Johanna Duffek of The International Dark-Sky Association, a group that campaigns against light pollution, advises making bedrooms as dark as possible.
If light is shining in your window use blackout curtains and turn off night lights, " she says.
"If you use a night light for your children try to use a red bulb to reduce the impact on melatonin production."