Women working the night shift more than twice a week for six years or more may have double the risk of breast cancer compared to those who work on day shifts, warn researchers.

The study, published in the journal Occupational and Environmental Medicine, found that the breast cancer risk was cumulative and four times greater in women who described themselves as "morning" rather than "evening" people compared to those who don't work night shifts.

While women who preferred to stay up late at night were less affected by night shifts, they were still twice as likely to have breast cancer compared to those who don't work night shifts.

Danish researchers analyzed medical records of 18,500 women who worked for the Danish Army between 1964 and 1999.

Researchers were able to contact 210 of the 218 women who had breast cancer between 1990 and 2003 and who were still alive in 2005. They matched these women with 899 women of the same age who had also worked for the Danish Army, but had not developed breast cancer.

After comparing the women's working patterns, lifestyles and other factors like participants use of contraceptives and hormone replacement therapy and sun tanning habits, they found that overall participants who worked at night had a 40 percent increase in breast cancer risk. 

Furthermore, women working night shifts at least three times a week for at least six years had double the cancer risk, according to the new findings.

The latest findings add to the mounting evidence that the disruption to the circadian rhythms and changes in levels of the 'darkness' hormone melatonin might be responsible along with sleep deprivation may boost cancer risk.

Researchers explain that exposure to light at night inhibits the body's production of the hormone melatonin, which is made by the pineal gland in the brain between the hours of 9pm and 8am. The hormone is responsible for controlling the natural cycles that govern sleep patterns and is believed to also help suppress tumors.

Past scientific findings suggest that unusually low levels of melatonin, generally seen in people exposed to light during the night, may promote tumor growth.

Researchers say that another explanation could be that sleep deprivation from working night shifts may lead to the suppression of the immune system, which might increase the growth of cancer cells.

Lead author Dr. Johnni Hansen of the Institute of Cancer Epidemiology, in Copenhagen, Denmark noted in the study that there appeared to be no increased risk for breast cancer in women working just one or two nights a week, suggesting that one or two night shifts will not change the timing of melatonin production and trigger circadian disruption.

However he warned that the latest findings may have underestimated the effect of night working on breast cancer because the study only included participants who had breast cancer and were still alive.

"The observation that women with night work and morning preference (who may be less tolerant of night shift work) tend to have a higher risk for breast cancer than similar women with evening preference warrants further exploration in larger studies," Hansen added, according to The Telegraph.

"Since night shift work is unavoidable in modern societies, this type of work should be limited in duration and limited to less than three night shifts per week," Hansen said, according to HealthDay. "In particular, morning types should limit their night work."