The study, published online in Occupational and Environmental Medicine, found the increased breast cancer risk seemed to be cumulative and strongest among those who describe themselves as "morning" people or "larks," rather than "evening" people or "nightowls."
Dr. Johnni Hansen, Institute of Cancer Epidemiology, Danish Cancer Society in Copenhagen, Denmark, said the study involved more than 18,500 women working for the Danish Army from 1964 to1999, all of whom had been born between 1929 and 1968.
Hansen and colleagues were able to contact 210 women out of a total of 218 who had had breast cancer between 1990 and 2003, and were still alive in 2005/2006.
These women were then matched with 899 women without breast cancer, Hansen said.
In all, 141 of those with breast cancer, and 551 of those free of the disease, completed a detailed 28-page questionnaire.
The results, based on 692 responses --141 from women with breast cancer -- showed that, overall, night shift work was associated with a 40 percent increased risk of breast cancer, compared with no night shifts.
The findings were published online in Occupational and Environmental Medicine.