Does working nights increase my risk of cancer?
There are indications that prolonged night work slightly increases your cancer risk, but the evidence is limited and contradictory. If you structurally work nights, it is sensible to factor this into conversations about preventive screening.
Night shifts have been linked to a slightly to moderately elevated cancer risk in multiple review studies. That risk ranges between 1% and 32% higher than in people who do not work night shifts. That wide range already says something: the individual studies frequently contradict one another, and a direct causal relationship has not been proven. For now, this remains a statistical association.
Most of the research concerns breast cancer. Women who work night shifts for more than 20 years, or for shorter periods but with many consecutive nights in a row, appear to have an elevated risk. For this reason, the International Agency for Research on Cancer (IARC) classified night work as 'probably carcinogenic' in 2007. But that classification rests on limited evidence: the studies use widely varying definitions of 'night shift' and their results differ considerably. This makes it impossible, for the time being, to make firm statements about the precise size of the risk.
The most commonly cited biological mechanism is the suppression of melatonin by artificial light at night. Melatonin, a hormone your body produces in the dark, may have tumour-inhibiting properties. Night work also disrupts your circadian rhythm more deeply, which can have further biological consequences. This sounds plausible, but has not yet been conclusively demonstrated in humans as a causal link.
What does this mean in practice? If you work night shifts occasionally, there is no cause for major concern. The risk that emerges from the studies is relatively small and applies primarily to prolonged and intensive night work over decades. If you have a job in which you structurally work nights, it is worthwhile to speak with your doctor about breast cancer screening and other lifestyle factors that influence your risk.
Based on meta-analyses and systematic reviews (PMID 27803010, 28770538, 31132107, 36834801). All findings are associative; the underlying studies are heterogeneous in definition and outcome. Causality has not been proven for cancer in general; for breast cancer after prolonged night work, the association is estimated as 'probable' (IARC 2007).