From a mail list:

In a study at Havard Medical School and B and W Hospital in Boston comes support of earlier research that women who work regular night shifts have a higher risk of breast cancer.

Women's cancers are often linked with estrogen, and melatonin may play a more important role than thought in the past. Data is beginning to show that it may be melatonin (not estrogen) is influencing cancer risk, reports Dr. Eva Schernhammer who led the study.

Melatonin is produced at night and regular exposure to sunlight affects production cycle. Artifical light suppresses its production. Melatonin has well established anticarcinogenic properties and the melatonin pathway could offer one explanation for the increased risk, the researchers noted, but urged further study.

The researchers studied 78,586 women who worked at least three night shifts per week for 15 yrs. These women had a 35% greater risk of colon or rectal cancer.

~Susan