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Night Shift Work and the Risk of Cancer

November 30th, 2007 | by Mark Sapienza, M.D. |

Working the night shift may increase a person’s risk of developing cancer.  A possible link between the development of certain types of cancer and night time shift work was first proposed in the 1980’s but dismissed as ridiculous.   It wasn’t until the analysis of recent studies, to be reported in this month’s Lancet Oncology by the The International Agency for the Research on Cancer (IARC),  a division of the World Health Organization (WHO),  that the idea gained credibility again.   Studies have shown an increase in breast cancer rates in women who work night shift over many years.   Similar observations have also been made in men with the development of prostate cancer and in animal studies where day night reversal leads to the development of more tumors and quicker deaths.   The observations are still being greeted with skepticism by the scientific and medical community and lack evidence beyond a casual association.   

Theories to why night time shift work may lead to cancer revolve around the role of melatonin;   a hormone produced by the brain usually during night time hours when people sleep.   The hormone may have a protective role in suppressing tumors and people who work night shifts may have lower melatonin levels.   Sleep deprivation and altered immunity may also be factors associated with the development of cancer.  

The IARC  is likely to list night shift work as a carcinogen in the next month.   The American Cancer Society is likely to follow with a similar advisory.   

Reference:  Straif K  The Lancet Oncology.   Dec 2007;  Vol 8(12)

Tags: Breast Cancer, Cancer, Prostate Cancer, sleep