Most studies on shift work and cancer have focused on breast cancer. There is some evidence that people who have worked night shifts for 20 to 30 years or more are at a slightly increased risk of developing breast cancer. The results are consistent with animal studies that have shown that disrupting the circadian system, or 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. However, the majority of studies that have found a link between cancer and shift work did not account for lifestyle choices that can affect a person’s cancer risk, for example how much alcohol they consume, whether they are overweight or obese or on hormone replacement therapy (HRT). The studies that did account for these other factors found a weaker link. Furthermore, different studies used varying definitions of ‘shift work’. There have been some studies linking shift-work with bowel, prostate and womb cancers, but the evidence is inconclusive. The International Agency for Research into Cancer (IARC) has classified shift work that involves circadian disruption as category 2A. The evidence of a link between working night shift and cancer risk is accumulating. However, if the effect is real, it is likely to be a small increase in risk. It remains possible that this is caused by other factors like those mentioned earlier. This means that there is some evidence that shift-working 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.