Graveyard shift: Night work linked to cancer

Working the night shift is emerging as a risk factor for cancer, a sleep expert has warned, adding to the work practice's long list of health woes.

People who work overnight were known to have higher rates of diabetes and obesity among other health problems, said Associate Professor Naomi Rogers, while evidence was emerging of a heightened risk of certain cancers.

It could be directly linked to the night shift's "disruptive" effect on the human body clock, said Dr Rogers, director of the Chronobiology and Sleep Research Group at the University of Sydney's Brain and Mind Institute.

"Shift workers seem to have an increased incidence of some types of cancer -- breast cancer, prostate, colorectal cancers and we're not sure why," Dr Rogers told AAP on Tuesday.

"With the shift work they get circadian disruption ... disrupting a whole range of hormones, but they also have sleep loss as well."

One of those hormones is melatonin, which Dr Rogers described as the "hands on the clock" of the body's circadian rhythm.

The hormone was known to boost the functioning of the immune system and play other beneficial roles, she said, along with helping the body to know what time it was.

"It's at low levels during the daytime and it starts to increase about two hours before sleep," Dr Rogers said of the hormone's production during a normal sleep cycle.

"It helps provide time information to the rest of the body to ensure everything happens in a co-ordinated fashion."

Tests in animals with cancer have shown that an injection of melatonin can slow down, or stop, the growth of a tumour.

Dr Rogers also said research animals that had no production of the hormone had an increased rate of tumour growth.

The production of melatonin in the brain's pineal gland was known to be suppressed by exposure to light -- and this includes the artificial light night-workers are exposed to.

"It is actually suppressed by light so the theory is shift workers who are awake instead of being asleep, their melatonin decreases," Dr Rogers said.

More research was needed to conclusively rule on the risk, she said, but night shift workers who had no other job options should take precautions.

"It's one risk factor -- and you should look at the other risk factors you have and take a healthy approach, so no smoking and eat well," Dr Rogers said.

"And then when you're not working, try to get good amounts of sleep."

Dr Rogers gave a presentation on sleep and cancer at the Clinical Oncological Society of Australia's annual scientific meeting, the nation's major gathering of cancer experts, which is under way this week on the Gold Coast.

- AAP