The threat rose with the more night shifts they did, the study found.
Night owls were twice as likely to have breast cancer, and overall there was a 40 percent bigger risk compared to women who worked days.
The results were based on 692 responses, of which 141 were from women with breast cancer. The risk almost quadrupled if they were early bird types possibly because they are more susceptible to body clock disruption.
Disturbing normal sleep patterns is thought to curb the cancer-protecting hormone melatonin, which is produced by the brain in the day.
The results indicate frequent night shift work increases the risk for breast cancer and suggest a higher risk with longer duration of night shift according to Dr Johnni Hansen, of the Danish Cancer Society that did the study.
"Those with morning preference tended to have a higher risk than those with evening preference," Dr Hansen stated.
The study also indicated working up to two night shifts a week had no impact as it may not be long enough to disrupt the body clock.