But study didn't prove cause-and-effect relationship.
Working at night might do more than throw your schedule out of whack, researchers warn. Women who cover the night shift may be at increased risk for ovarian cancer, according to a new study.
The investigators also found that the risk of ovarian cancer may be lower for night types ("owls") than for morning types ("larks").
How the study was done
The study included more than 1 100 women with the most common type of advanced ovarian cancer, including about 390 with borderline disease, and more than 1 800 women without ovarian cancer.
The women were aged 35 to 74, and worked in fields including health care, food service and office administration.
Nearly 27% of the women with invasive cancer had worked nights, compared with about 32% of those with borderline disease and about 22% of those without ovarian cancer, the findings showed.
Working night shifts was associated with a 24% increased risk of advanced cancer and a 49% increased risk of early stage cancer, according to the study.
Only women 50 and older were significantly more likely to have ovarian cancer if they had worked nights, study author Parveen Bhatti, of the Fred Hutchinson Cancer Research Center in Seattle, said in a news release.
What the study found
Of the women in the study who had worked night shifts, 27% said they tended to be night owls and 20% said they were morning larks. The risk of advanced ovarian cancer was higher among larks than among owls (29% versus 14%), as was the risk of borderline disease (57% versus 43%).
The researchers suggested that the increased cancer risk in women who work night shifts could be associated with the hormone melatonin, which regulates reproductive hormones, particularly oestrogen. Melatonin is normally produced at night but is suppressed by light.
Previous research has suggested that night-shift work may be associated with an increased risk of breast cancer.
Although the study found an association between night-shift work and increased risk of ovarian cancer, it did not prove a cause-and-effect relationship.