Work at night and breast cancer – report on evidence-based options for preventive actions

In 2007, the International Agency for Research on Cancer classified shift work involving circadian disruptionas probably carcinogenic to humans (group 2A), primarily based on experimental and epidemiologic evidence for breastcancer. In order to examine options for evidence-based preventive actions, 16 researchers in basic, epidemiological andapplied sciences convened at a workshop in Copenhagen 26-27 October 2011. This paper summarizes the evidence fromepidemiological and experimental studies and presents possible recommendations for prevention of the effects of nightwork on breast cancer. Among those studies that quantified duration of shift work, there were statistically significantelevations in risk only after about 20 years working night shift. It is unclear from these studies whether or not thereis a modest but real elevated risk for shorter durations. Hence, restriction of the total number of years working nightshift could be one future preventive recommendation for shift workers. The diurnal secretion of melatonin by the pinealgland with peak in secretory activity during the night is a good biochemical marker of the circadian rhythm. Disruptionof the diurnal melatonin secretion pattern can be diminished by restricting the number of consecutive night shifts.Reddish light and reduced light intensity during work at night could potentially help diminish the inhibitory activityof light with strong intensity on the melatonin secretion, but further mechanistic insight is needed before definiterecommendations can be made. Earlier or more intensive mammography screening among female night shift worker is notrecommended because the harm-benefit ratio in this age group may not be beneficial. Preventive effects ofmelatonin supplementation on breast cancer risk have not been clearly documented, but may be a promising avenue if alack of side effects can be shown even after long-term ingestion. Women with previous or current breast cancer shouldbe advised not to work night shifts because of strong experimental evidence demonstrating accelerated tumor growth bysuppression of melatonin secretion. Work during the night is widespread worldwide. To provide additional evidence-basedrecommendations on prevention of diseases related to night shift work, large studies on the impact of various shiftschedules and type of light on circadian rhythms need to be conducted in real work environments.