Viral infections more dangerous during morning: Study

Disruptions in body clock lead to increased virus replication and dissemination, indicating that severity of acute infections is influenced by circadian time-keeping. 

"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, Professor at University of Cambridge.

For the study, the team compared normal 'wild type' mice infected with herpes virus and influenza A virus at different times of the day, measuring levels of virus infection and spread.

The mice lived in a controlled environment where 12 hours were in the daylight and the other 12 hours were dark.

The results showed that virus replication in those mice infected at the very start of the day, when these nocturnal animals start their resting phase, the risk of infection was shown to be ten times greater than those infected by the end of the day while they were transitioning to their active phase. 

Abolishing cellular circadian rhythms increased both herpes and influenza A virus infection in the mice, the researchers said.

"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," added Rachel Edgar from University of Cambridge. 

"This indicates that shift workers, who work for some nights and rest for other nights and 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," Rachel noted.

In addition, Bmal1 -- a gene that controls the circadian rhythm -- also undergoes seasonal variations. It remains less active during winter, while it increases activity in summer, thus explaining the reason why diseases, such as influenza, are more likely to spread throughout populations during winter, said the paper published in the journal Proceedings of the National Academy of Sciences (PNAS).