Two independent US studies published in October 2001 in the Journal of the National Cancer Institute concluded that sleep interruption, especially in women working the graveyard shift, is associated in an increased risk of breast cancer. Working the graveyard shift increased the risk of breast cancer by 60 per cent, Dr Scott Davis and colleagues found, adding that the risks increased with increasing years and weekly hours working these shifts.  In the second study, Dr Eva S Schernhammer and colleagues found an 8 per cent increase in breast cancer risk among nurses working rotating night shifts for up to 29 years. The women who had worked these shifts for more than 30 years had a 36 per cent increase in breast cancer risk. In a related editorial, Dr Johnni Hansen – whose later work, particularly papers published in 2010, would have a serious impact on the recognition of shift related breast cancer – noted that work at night was an increasing phenomenon, more commonplace than any other exposure to a known or potential cancer-cause. Over a decade after the 2001 studies, the UK safety regulator HSE was still facing criticism for a failure to act what became an established cancer risk factor.

Davis S and others. Night shift work, light at night, and risk of cancer, Journal of the National Cancer Institute, volume 93, number 20, pages 1557-62, 2001.
Schernhammer ES and others. Rotating shifts and risk of breast cancer in women participating in the Nurses’ Health Study, Journal of the National Cancer Institute, volume 93, number 20, pages 1563-8, 2001.
Hansen J. Editorial: Light at night, shiftwork, and breast cancer risk, Journal of the National Cancer Institute, volume 93, number 20, pages 1513-5, 2001.