Can working night shifts cause cancer?

In 2007, the International Agency for Research on Cancer (IARC) classified shift work that follows an irregular time pattern as a probable cause of cancer. This was based on some animal and human studies that found that people who worked shifts or during the night were more likely to develop some types of cancer, such as breast and prostate cancer.

It also came from the idea that not following the natural day/night cycle and disrupting that natural rhythm in your body, called your ‘circadian rhythm’, could have a negative impact on health. This is somewhat true, as there are biological processes that are triggered by exposure to daylight, or by hormones that fluctuate over the course of the day.

Night shift impact on health

People that work shifts are also less likely to get enough sleep and more likely to be overweight or obese, both of which can have a significant effect on your overall health. Night shift work can make it harder to look after your general health – people tend to feel more tired when awake during the night, so finding time to get enough exercise or cook healthy food can be difficult. This means there are many health factors that could also increase your risk of cancer that are not specifically caused by working at night.

More recent studies on night shift work and cancer risk factor in information like this, such as BMI and waist size, and have not found that shift work itself makes someone more likely to cause cancer. In 2016, a meta-analysis (which combines the data of many different studies together) found no link between night shift work and breast cancer. In 2020, another meta-analysis that included data from 57 different studies and at least nine different types of cancer found no increased risk with night shift work.

So no, night shift work does not itself increase your risk of developing cancer. Instead, this type of working pattern may lead to other health behaviours or factors that increase your risk, such as being overweight or obese, or not doing enough physical activity.

This means that, if you do work night shifts, the best way to reduce your cancer risk is to look after your general health:

• Eat a balanced diet with plenty of fruit, vegetables, and fibre, and limit your consumption of red and processed meats
• Moderate how much alcohol you drink
• Don’t smoke!
• Get regular exercise that gets your heart pumping and some resistance training