Night shift work and stomach cancer risk in the MCC-Spain study. 
Night shift work has been classified as a probable human carcinogen by the International Agency for Research on Cancer, based on experimental studies and limited evidence on human breast cancer risk.  Evidence at other cancer sites is scarce.  We evaluated the association between night shift work and stomach cancer risk in a population-based case-control study.  A total of 374 incident stomach adenocarcinoma cases and 2481 population controls were included from the MCC-Spain study.  Detailed data on lifetime night shift work were collected including permanent and rotating shifts, and their cumulative duration (years).  Adjusted unconditional logistic regression models were used in analysis.  A total of 25.7% of cases and 22.5% of controls reported ever being a night shift worker.  There was a weak positive, non-significant association between ever having had worked for at least 1 year in permanent night shifts and stomach cancer risk compared to never having worked night shifts (OR=1.2, 95% CI 0.9 to 1.8).  However, there was an inverse 'U' shaped relationship with cumulative duration of permanent night shifts, with the highest risk observed in the intermediate duration category (OR 10-20 years=2.0, 95% CI 1.1 to 3.6) (p for trend=0.19).  There was no association with ever having had worked in rotating night shifts (OR=0.9, 95% CI 0.6 to 1.2) and no trend according to cumulative duration (p for trend=0.68).  We found no clear evidence concerning an association between night shift work and stomach cancer risk.