Working the night shift a few nights a month can do more than leave your eyes bleary-eyed ,suggests a study on Wednesday.     It just might increase your risk of color1ectal cancer .Nurses who worked the night shift at least three times a month for t5 years or more were 35%more likely to develop that type of cancer than nurses who never worked nights,Harvard University researchers report in the Journal of the National Cancer Institute.     The finding,from the ongoing Nurses Health Study,is the first to link night shift work with a higher risk of color1ectal cancer,expected to kill 57,100 Americans this, according to the American Cancer Society.Another research,including a report out of the Nurses Health Study, has suggested that working night shifts increases women's breast cancer risk.     The researchers asked 78,586 nurses whether they had ever worked three or more night shifts a month.The nurses indicated whether they had done so for less than 15 years or 15 years or more.The scientists followed the women to see who developed color1ectal cancer.     Researchers believe that lowered levels of melatonin might increase night-shift workers' cancer risk.Melatonin levels usually reach the highest point in the middle of the night. Turn on the lights,though,and  _ dropped measurably in just 10 minutes,says Richard Stevens a cancer epidemiologist at the University of Connecticut who did not take part in the new study.After two weeks of continuous nightly exposure to light,humans experience a sharp drop in melatonin production,Nurses Health Study researchers write.    In the lab,melatonin appears to stop tumor growth.Other studies suggest that color1ectal cancer patients have lower blood levels of melatonin than healthy people,the authors write.However,that could be a result of the disease,not a cause of it.Stevens says. Which of the following plays an important part in causing cancers for nurses working night shifts?
Blood levels of melatonin Tumor growth The exposure to light Melatonin production.