Women who regularly work into the early hours can be nearly four times as likely to develop breast cancer, scientists have warned.
The risk is highest among women who are naturally early risers. But even night owls are in danger.
The threat rose with the more night shifts they did, the study found.
And overall there was a 40 percent bigger risk compared to women who worked days.
“The results indicate frequent night shift work increases the risk for breast cancer and suggest a higher risk with longer duration of night shift,” the Mirror quoted Dr Johnni Hansen, of the Danish Cancer Society that did the study, as saying.
“Those with morning preference tended to have a higher risk than those with evening preference,” Dr Hansen stated.
Women who worked nights three or more times a week for over six years were more than twice as likely to have the disease as those who had not.
The risk almost quadrupled if they were early bird types – possibly because they are more susceptible to body clock disruption, said the study.
Night owls were twice as likely to have breast cancer, according to the findings published online in Occupational and Environmental Medicine.
The results were based on 692 responses, of which 141 were from women with the disease.
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.
Disturbing normal sleep patterns is thought to curb the cancer-protecting hormone melatonin, which is produced by the brain in the dark.