Many years of work during the night or early morning increases the risk of certain cancers. Night work is likely to increase the risk of breast cancer and may increase the risk of prostate cancer.
Around 20% of all work in the western world is done in the evening, at night or early in the morning. People undertaking this type of work have their circadian rhythms disrupted because they are exposed to light at a time when it naturally is dark. Furthermore, a person generally sleeps less when working at night.
The risk of cancer owing to prolonged night work may be because the brain produces less of the hormone melatonin, which inhibits the growth of cancer cells.