Schernhammer et al.(8) studied the association between night work and breast cancer among nurses participating in the Nurses Health Study and followed up in 1988-1998. Based on a question in 1988 rergerding how many years in total the woman had worked rotating night shifts with at least 3 nights per month, a significant increase of risk (relative risk=1.36) was found for the longest exposure (มากกว่าหรือเท่ากับ30 years), compared with never working such shifts. In the Nurses' Health Study II