The findings could also explain why shift workers are more prone to infections and chronic disease.

Scientists test our body clock's susceptibility to infection 

Viruses are more dangerous when they infect their host in the morning, according to new research.

A study by the University of Cambridge found that people are 10 times more susceptible to infection at the very start of the day than in the evening.

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

They found that virus replication in mice infected at the very start of the day was ten times greater than mice infected 10 hours into the day.

Professor Akhilesh Reddy from the University of Cambridge explains: “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 can cause a much more severe acute infection.”

Published in the journal Proceedings of the National Academy of Sciences, the findings could also explain why shift workers are more prone to infections and chronic disease.

Focusing on Bmal1 - a gene that regulates the body clock - the researchers found high levels of virus replication, regardless of the time of infection.

“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,” said Professor Reddy.  

The researchers also found that Bmal1 undergoes seasonal variations. It is less active in the winter months, increasing in the summer. The researchers suggest this might explain why infections like influenza are more likely to spread in winter. 

The study, Cell autonomous regulation of herpes and influenza virus infection by the circadian clock, is published in the journal Proceedings of the National Academy of Sciences.