A new long-term study has reported that the increased risk of breast cancer previously reported to be associated with night work may be related to the number of consecutive night shifts worked. Occasional night shifts, even over of a period of decades, appears to have little influence on risk of breast cancer. However, working five or six consecutive night shifts over a period of years heightens risk. It has been proposed that the higher breast cancer risk associated with night work may be the result of mechanisms influenced by the perception of light by the eye, such as melatonin or circadian synchronization. Previous population studies have found a relationship between night shift work and melatonin production.

The present study included 44,835 Norwegian nurses. A total of 699 breast cancer cases diagnosed between 1990 and 2007 (representing 74% of the breast cancer cases found in the 44,835 women) and 895 cancer-free controls were interviewed to obtain data concerning work history and potential breast cancer risk factors.

No increase in breast cancer risk was observed after long periods during which nurses worked at least three night shifts per month. Small, statistically insignificant increases in risk were found for exposure to 30 or more years of work in hospitals or other institutions, working at least 12 years in schedules including some night shifts, working a total of at least 1,007 night shifts during one's lifetime, and lifetime average of at least four night shifts per month. Higher, but still nonsignificant, risks of breast cancer were also observed in women who worked at least five years with at least four consecutive night shifts. However, significantly increased risks of breast cancer were found in nurses who worked at least five years with six or more consecutive night shifts. The authors conclude that breast cancer risk might be related to number of consecutive night shifts.