Scientists suggest that viruses are more dangerous if the infection occurs in the morning.
After conducting animal studies, researchers from the University of Cambridge found that if viral infections occurred in the early hours of the day, the virus is 10 times more successful. Moreover, the researchers found that a disrupted body clock could also increase vulnerability to infections.
The researchers studied laboratory mice infected with either influenza virus, which causes flu, or the herpes virus, which causes a range of diseases including cold sores. They found that mice infected in the morning had increased viral levels 10 times than those that were infected in the evening.
"It's a big difference," Akhilesh Reddy, a senior fellow at the University of Cambridge's Department of Clinical Neurosciences and co-author of the study, told BBC News. "The virus needs all the apparatus available at the right time, otherwise it might not ever get off the ground, but a tiny infection in the morning might perpetuate faster and take over the body."
Viral infections occur when viruses enter the cells in the body and reproduce. However, cells follow the circadian clock or the body's natural clock, and they coordinate with daily environmental changes during the day or night. This means that if the infections happen late in the evening when the cells are less active, then the viral levels could be lower.
Reddy added that the findings of the study, which was published in the Proceedings of the National Academy of Sciences, could also help in controlling viral outbreaks.
"In a pandemic, staying in during the daytime could be quite important and save people's lives, it could have a big impact if trials bear it out," Reddy said.
Furthermore, the researchers found that disruption of the body clock or the circadian rhythm meant that the body was "locked in" to a state that allowed the viruses to reproduce.
"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," Rachel Edgar, a research associate at University of Cambridge and first author of the study, said in a statement.
"If so, then they could be prime candidates for receiving the annual flu vaccines."
The researchers used two viruses in the study, a DNA virus and an RNA virus. The study also focused on one clock gene, which is called Bmal1, which has a peak activity in the afternoon in both mice and humans.
"It's the link with Bmal1 that's important, since when that's low [in the early morning], you're more susceptible to infection," Reddy added.