Night shift work may stop the body repairing daily damage to DNA and raise the risk of mutations which lead to cancer, a new study suggests.

The link between working at night and poor health has been known for several years, with those who work after dark more likely to suffer diabetes, obesity, poor fertility, heart attacks and tumours.

Scientists believed that disruption to the body’s natural body clock was responsible for the increased risk of chronic illness, but could never pinpoint the mechanism.

Now US researchers have discovered that when people work nights they produce 80 per cent less of a chemical which is a by-product of DNA tissue repair. They say it indicates that the body is not carrying out the crucial restoration to cells which should happen naturally overnight.

They believe the effect could be caused by a lack of melatonin, the sleep hormone, which is far lower among people who sleep in the daytime.