As many as 20% of people in Europe and the U.S. work the night shift. Exactly how night shift work may increase breast cancer risk is not clear. In 2007, the International Agency for Research on Cancer concluded that shift work disrupting the body clock or circadian rhythms may cause cancer. Such night work reduces production of the hormone melatonin.
Working up to two night shifts per week did not influence breast cancer risk. This may not be long enough to disrupt the body clock, study authors conclude. What's more, risk was also higher among self-described "morning" people, or "larks," who were working the night shift. Larks may be more susceptible to body-clock disruption than owls.
The night workers in the new study tended to sunbathe more frequently than their counterparts who worked by day. This observation does not support the theory that the increased breast cancer risk among night shifters was due to less sun exposure.
Several studies suggest that night-workers have a higher incidence of breast cancer -- as well as an association with more risk factors for breast cancer (e.g. obesity, smoking, alcohol use)," says Marisa Weiss, MD, via email. She is the president and founder of Breastcancer.org and director of breast radiation oncology and breast health outreach at Lankenau Medical Center in Wynnewood, Pa.
"It's probably the combined effect of getting less quality sleep -- fewer hours, with a disruption of circadian rhythms, more light at night -- and lower levels of melatonin, which is believed to play a role in regulating normal cell growth," she says. "Since the biggest risk factor for breast cancer in women is aging, it's not surprising that getting less quality rest may play a role, since that's the key time each day that we have to repair aging effects, i.e. the wear and tear of everyday living."