MONDAY, July 1 (HealthDay News) -- 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
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
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.
To learn more about shift work and health, visit the
U.S. Centers for Disease Control and
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