But it comes on the heels of another study released by the Danish Cancer Society in May which showed regular night-shift workers could face an increased risk of developing cancer.

This study echoed findings in other studies, including the World Health Organisation's cancer research agency. The Danish study warned that women who work frequent night shifts faced a 40 per cent increased risk of breast cancer. ''The results indicate that frequent night shift work increases the risk for breast cancer and suggest a higher risk with longer duration of intense night shifts,'' the study said.

In 2007, the International Agency for Research on Cancer listed shift work that involved night work as a probable carcinogen after several studies revealed an increased level of breast cancer in night workers. Britain's Health and Safety Executive has also commissioned an investigation after the Danish study.

But the controversial findings of the Australian study - the first biological study in the field and one of several studies that has found no significant risk - will challenge the World Health Organisation's findings when it is published this year.

''There is an association but I wouldn't be worried if I were a shift worker,'' the study author, Professor Lin Fritschi, said. Previous studies linked exposure to light at night and disruption of the body's circadian rhythms as the possible cancer causing agents.

Animal studies had shown that mice that were subjected to shift work conditions and bright light when they were meant to be sleeping developed tumours. This evidence, supported by limited evidence from human studies, prompted the Danish government to automatically compensate women who worked for more than 20 years in night jobs and got breast cancer.

But Professor Fritschi said her research ''found no evidence that that is actually a sensible policy because we didn't find any associations with that''.

The study looked at the life history of 1205 women with cancer and 1789 without, including 250 with breast cancer who had worked night shift for between one and 20 years. It found no evidence to support either the light at night or disruption of the circadian rhythm hypothesis. Professor Fritschi says the problem with previous studies is that they did not examine the factors from a biological basis.

''None of them have thought about how shift work might be associated with breast cancer. They just looked at the number of years, or number of shifts, or number of shifts people are doing shift work,'' she says.

''We think our study is better because we had these really strong hypotheses and had a range of women doing lots of different types of shiftwork.''

Professor Fritschi said the only factor that returned a slightly higher risk was in the number of nights shifts you do in a row. ''If anyone wanted to change something … do fewer nights in a row.''