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.
Dr Parveen Bhatti, Fred Hutchinson Cancer Research Center, Seattle, Washington, USA, said if awake at night the body has "reduced capacity to repair and clear oxidative DNA damage."
The study tested 50 night shift workers for levels of 8-OH-dG - a chemical which is produced when DNA is repaired.
They then tested them again when they were working days, and found levels jumped by 300 per cent.
They believe that shift workers may need to take sleep hormone supplements to allow DNA to carry out repairs as they sleep in the day.
"If such effects are confirmed, melatonin supplementation should be explored as an intervention to reduce the occurrence of potentially carcinogenic DNA damage among shift workers," added Dr Bhatti.