Most studies on shift work and cancer have focused on breast cancer and there is some evidence that people who have worked night shifts are at a slightly increased risk of developing breast cancer. The results are consistent with animal studies that have shown that disrupting the 24-hour body clock, can increase the risk of cancer. Being exposed to artificial light at night can alter sleep patterns, suppress melatonin production – which is thought to help prevent cancer – and affect genes involved in tumour development.
There are some studies of prostate cancer, and bowel cancer with inconclusive results. The evidence of a link between working night shift and cancer risk is accumulating but there is a lot of variation in how the studies are conducted and the results are inconsistent between studies. If the effect is real, it is likely to be a small increase in risk and may be caused by other factors such as body weight or physical activity. The International Agency for Research into Cancer (IARC) has classified night shift work as category 2A (probable carcinogen). This means that there is some evidence that night shift work could affect the risk of cancer, but that more research is needed to say for sure. IARC identified a need for more studies to examine the potential risk in different professions and for other cancers.