A new study published in Proceeding of the National Academy of Sciences (PNAS) suggests that viruses are 10 times more likely to be successful at infecting their hosts in the morning. Research also found that those who had a disrupted circadian rhythm because of the pattern of their jobs, jet-lag, or disrupted sleep patterns in general may be even more susceptible to infection.
The study infected mice with a virus at different points during the day and it was found that the virus was 10 times more likely to take when they were injected with the virus in the morning. This new research could help scientists figure out how viruses spread from one person to the other and may be able to stop the spread of pandemics and epidemics.
One of the lead researchers of the study, Professor Akhilesh Reddy said:
“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. 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.” 
The BBC news likened viruses trying to attack later in the day and failing as attempting to hijack a factory when all of the workers had gone home for the day.
However, researchers only studied two viruses, so it is currently unknown whether the study is specific to these diseases or to every virus. But since they were working with a distinct RNA virus and DNA virus, scientists feel that this may apply to a broad range of viruses.
Here is what the study abstract has to say:
“The circadian clock coordinates our physiology. Circadian disruption, as occurs during shift work, increases the risk of chronic diseases. For infectious diseases, circadian regulation of systemic immunity seems to underpin “time-of-day” differences in responses to extracellular pathogens.
However, circadian rhythms are cell autonomous, and their interaction with intracellular pathogens, such as viruses, is poorly understood. We demonstrate that time of day of virus infection has a major impact on disease progression, in cellular models as well as in animals, highlighting the key role that cellular clocks play in this phenomenon. Clock disruption leads to increased virus replication and dissemination, indicating that severity of acute infections is influenced by circadian timekeeping.”
The research also zeroed in on an internal clock called Bmal1, which is most active in the afternoon. It is also less active in the winter, which may explain why more people get sick during the cold months.