CLOCK and breast cancer: Breast cancer is the most frequent cancer among females and the incidence is higher in industrialised compared with low-income and middle-income countries.1 The increase seen in industrialised societies has mainly been attributed to reproduction and lifestyle factors.2 3 However, the use of artificial light during work at night, leading to disruption of the internal circadian clock and suppressed melatonin production, may accelerate the transformation of normal cells to malignant mammary cells and promote the risk of breast cancer.4 5