Viruses are more effective when they infect victims in the morning, a study has suggested.
The study, published in the journal PNAS, showed that mice infected in the morning had 10 times higher viral levels than those infected in the evening.
It also found that mice whose body clock was disrupted were more vulnerable to infection at any time.
Viruses can only spread in the body by hijacking cells as they do not have the chemical machinery to survive on their own.
But these cells follow a 24-hour circadian rhythm commonly known as the body clock.
Researchers at the University of Cambridge focused on one gene, Bmal1, controlled by the body clock, which has its peak activity in the afternoon both in mice and in people.
Professor Akhilesh Reddy, one of the authors, told BBC News: ‘It’s the link with Bmal1 that’s important, since when that’s low [in the early morning], you’re more susceptible to infection.’
He said the findings could help control outbreaks of disease. ‘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 authors said the study also had implications for those who did night-shift work and whose circadian rhythm was likely to be disrupted.
This study revolves around an area of medicine which is increasingly becoming discussed: how the time of day, and the circadian rhythm, is important not only in the maintenance of health and the development of disease, but also for the effectiveness of medications. Most studies are conducted at particular times of the day to investigate the efficacy of medications, and there have been suggestions that the body may work in different ways at different times, making a medication more or less effective as a result.
In this example, they looked at the time of day of viral infection in mice, on the premise that the body is more effective and the immune system more active at some points in the day versus others. Specifically, when the body is resting it is technically is more vulnerable than when it is fully alert and working well. The suggestion is that when the circadian rhythm is disrupted, this disrupts and suppresses systems such as immunity.
Two different types of mice were infected with an intranasal herpes virus at two different times of day, and then a viral load, or count, was taken to determine the effect. Biochemical samples were also taken from the mice to deduce the level of biochemical signalling related to their circadian rhythm. One of the groups of mice did not have one of the chemicals responsible for the circadian rhythm and therefore would not have a circadian rhythm as such.
The viral loads were much higher in the mice infected earlier in the day compared with those infected 10 hours later on. The higher viral loads were comparable to those found in mice without a circadian rhythm.
Subsequently, the authors sought to prove that this happened with other viruses, and instead used herpes simplex via a scratch on the ear to inoculate the mice. The investigators used a bioluminescent protein to show the production of the virus in the mouse. The amounts of HSV in the mice were greatly increased in those mice without a circadian rhythm.
The presence of a circadian rhythm is also noted in cells alone. Therefore the researchers tested the theory on cells, again, replicating the results seen in the mice.
In discussion, the authors sought to propose explanations behind this effect, the main one being that desynchronised cells tend to be more susceptible to viral infection and viruses tend to be more expressed in these cells as a result. This could have wide-ranging health implications, from public health planning in the event of a pandemic, through to the health implications of shift work and the 24-hour culture we are increasingly subject to. It could even be a mechanism for the development of cancer — as has been shown before in night-shift workers with breast cancer, I believe.
I think in the future circadian rhythm and the importance of regular, quality sleep will be shown to have significant impact on public health and on the modification of risk for developing many diseases. This study is proof of concept for viral susceptibility in mice at least.
Research score: 4/5