Working the night shift for 30 years or more may double the risk of breast cancer, a new Canadian study suggests.
The study found an apparent connection between night-shift work and breast cancer risk, but it did not prove the existence of a cause-and-effect relationship.
Other research has also found a link between night-shift work and breast cancer, especially for health-care professionals. But the new study revealed an apparent risk among other types of workers, said lead researcher Kristan Aronson, a professor of public health sciences at the Queen's Cancer Research Institute at Queen's University in Kingston, Ontario.
While those women with 30 or more years of night-shift work had a doubling of risk, Aronson's team found no increased risk among those who worked nights for less than 30 years.
The researchers obtained very specific details about the women's work history.
"We were very careful in asking about lifetime occupational histories, including specific start and stop times of each shift worked," Aronson explained, "so we carefully assessed each woman's exposure to night work."
Aronson's team looked at more than 1,100 women with breast cancer and more than 1,100 others without the diagnosis who were the same ages and lived in Vancouver or Kingston. On average, the women were in their mid to late 50s. Some had already gone through menopause, while others had not.
Women answered questions about their work patterns. The researchers got information about cancer diagnoses from hospital records. About one-third of the women in each group were ever involved in night-shift work.
When Aronson looked at the groups of women in terms of duration of night-shift work, she found that the link between working 30 years or more and a doubling of breast cancer risk held even after taking into account other factors that can affect cancer risk, such as body-mass index.
"There are only hypotheses so far about what could link long-term shift work to increased breast cancer risk," she said. "Some hypotheses are: disruption to the normal daily body [circadian] rhythm, decreased melatonin [a hormone that regulates sleep-wake cycles that is produced in greater quantity during sleep], increased sleep disturbance and possible lower vitamin D."
The study was published online July 1 in the journal Occupational and Environmental Medicine.
The findings echo those of previous research, said Russel Reiter, a researcher at the University of Texas Health Science Center in San Antonio, who has also studied the topic.
"The strength of the study is in the number of individuals included," he said. "Overall, it strengthens the association between night-shift work and breast cancer."
Among the possible explanations, he agreed, is suppression of melatonin. Night-shift work can affect melatonin levels. Melatonin may also help strengthen the immune system, some experts believe.
Besides preventive care such as mammograms, what can women do if they work night shifts? Many women don't have a choice of which shift to work, of course. If possible, Aronson said, women might try to work less than 30 years of night shifts.