Why shift workers may be prone to infections

The time of day we are exposed to a virus may affect how we respond to it and how sick we become, a new study by scientists at the University of Cambridge has found.

The research could explain why shift workers appear more prone to infections and chronic disease than regular daytime workers.

“The time of day of infection can have a major influence on how susceptible we are to the disease, or at least on the viral replication, meaning that infection at the wrong time of day could cause a much more severe acute infection,” said Akhilesh Reddy, senior author of the study published in the Proceedings of the National Academy of Sciences.

“This is consistent with recent studies which have shown that the time of day that the influenza vaccine is administered can influence how effectively it works.”

Circadian rhythms – our body clocks – control many of our bodily functions including our immune systems and the release of hormones.

That means that resources available to cope with a viral infection vary during the course of a day.

To test the impact of this on the progression of infection, the researchers put a group of mice in a controlled environment where they lived 12 hours in daylight and 12 hours in the dark.

They then infected the mice with the herpes virus at different times of the day, measuring levels of virus infection and spread.

Virus replication in mice infected at the beginning of the daylight period (when the nocturnal animals were entering their rest period) was 10 times greater than in mice infected two hours before they entered darkness – the active phase of their day.

When the researchers repeated the experiment in mice without Bmal1 – one of the genes that control circadian rhythms – virus replication was high regardless of the time of infection.

“Each cell in the body has a biological clock that allows them to keep track of time and anticipate daily changes in our environment,” says Rachel Edgar, the first author of the study.

“Our results suggest that the clock in every cell determines how successfully a virus replicates. When we disrupted the body clock in either cells or mice, we found that the timing of infection no longer mattered – viral replication was always high. 

“This indicates that shift workers, who work some nights and rest some nights and so have a disrupted body clock, will be more susceptible to viral diseases. If so, then they could be prime candidates for receiving the annual flu vaccines.”