Exposure to light-at-night, including disturbance of the circadian rhythm, possibly mediated via the melatonin synthesis and clock genes, has been suggested as a contributing cause of breast cancer.

Persons who engage in nightshift work may exhibit altered nighttime melatonin levels and reproductive hormone profiles that could increase the risk of hormone-related diseases, including breast cancer. Several studies have been conducted worldwide recently to investigate the effects of factors that can disrupt circadian rhythm and alter normal nocturnal production of melatonin and reproductive hormones of relevance to breast cancer etiology. We selected 5 international study, published on well known journals and based on large numbers of subjects, and we evaluated them in order to define a state of the art of this topic. Comparing these studies we conclude that nurses who frequently attend night shifts should undergo strict and periodical breast cancer screening, since the risk is significantly increased in this population.