Working night shifts may increase the risk of breast cancer, a conference on the health effects of light was told. 

Participants attending the National Research Council's symposium in Ottawa are reviewing research on how light alters the body.

The subject arose after the discovery of a unique photoreceptor cell in the retina six years ago.

The photoreceptor sends signals to the brain to say it's light or dark, although we don't report it as a visual sensation, said Jennifer Veitch of the council.

Instead, the information is sent to our biological clock, which regulates day-night cycles and orders the body to produce the hormone melatonin at night.

When people such as night shift workers and insomniacs are exposed to light during the night, they produce less or sometimes no melatonin — a hormone that has been shown to help prevent breast cancer.

Higher cancer risk for shift workers

Female night shift workers show a 50 per cent to 80 per cent higher risk of breast cancer, apparently because of light at night suppressing production of melatonin, said Dr. David Blask, a cancer researcher at the Mary Imogene Bassett Hospital in New York.

Blask, who did the research, presented the findings at the conference on Thursday. The meeting ends Friday.

Light may affect other cancers as well, he said. Blask proposed that melatonin supplements or fewer and shorter night shifts are potential solutions, but oncologists caution against jumping to the conclusion that melatonin may help reduce cancer since the idea hasn't been tested.

Along with concerns about too much light at night, there are also worries about not getting enough sunlight during the day.

Many windowless offices are bathed in florescent light, which may affect our moods and immune system.

Architects may need to re-learn how to design buildings to bring in daylight, said lighting designer Naomi Miller. The trick is to do it without increasing glare, she said.

Much remains to be learned about how light is interpreted by our brains, but scientists are concluding it may be healthy to stay in the dark at night and seek out sunlight during the day.