The time of day you contract a virus may influence how susceptible you are to developing an infection. Researchers found that when live animal models and human cell cultures were infected with Influenza A or herpes in the morning, they had up to 10 times more viral replication than when they were infected later in the day, greatly increasing the odds they would then develop the illness.

The new discovery could have a significant impact on how public health bodies respond to disease outbreaks, as well as for shift workers who have a disrupted body clock. This is because the researchers suspect that it could be our circadian rhythm that is dictating how susceptible our cells may be to infection, as the availability of certain cell resources fluctuates with our internal body clock. They tested this by seeing how easily viruses replicated in mice that had a circadian rhythm gene (Bmal1) knocked out, and found high levels of replication regardless of time of day.

“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,” explains Professor Akhilesh Reddy, senior author of the study published in PNAS. “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.”

The research could have important implications for how we respond to disease outbreaks. By knowing that the body is more susceptible to viral infection in the morning, it could mean that citizens could be asked during outbreaks to stay indoors until later in the day, when the level of viral replication may drop by as much as 10 times. 

Not only that, but the paper also found that those mice that had a disrupted body clock, something frequently seen in passengers after long flights or those who undertake shift work, were almost always vulnerable to infection. This could mean that those who have to work some nights but not others could be the ones who most need to get their yearly flu vaccinations, in order to protect their weakened immune system.

The fact that the researchers conducted the experiments using two viruses from the main viral families, one based on DNA and the other on RNA, seems to suggest that the effect applies across many different viruses and has a wide application.