Does time of day influence our susceptibility to infection?
"Viruses more dangerous in the morning," BBC News reports, but The Telegraph tells us that "the evening commute is worse for health".
So who's right? It depends if you're talking about mice or humans. What we do know is shift workers may be at added risk of catching a viral infection.
The apparently conflicting headlines were prompted by a UK study that aimed to find out if the time of day contact is made with a virus has an impact on how much and how quickly it spreads.
When mice were given the virus at the start of the day at the beginning of their daily resting phase, it reproduced 10 times more than in mice infected 10 hours into their active phase.
The body clock is said to have an effect on the body's cells. And because viruses make use of our cell activity to spread, the researchers feel viruses may use this to their advantage.
But a point that some of the media seemed to have missed is that mice are nocturnal animals – so their morning, when their body clock "winds down", is equivalent to the evening for humans.
Researchers note that shift workers may be at increased risk of infection as a result of their body clock being disrupted.
Obviously, human cells are not identical to mice and the findings may not be directly applicable to humans.
Still, there may be a case for adding shift workers to the list of vulnerable people who should receive the annual seasonal flu jab.
Where did the story come from?
The study was carried out by researchers from the University of Cambridge.
Funding was provided by the Wellcome Trust, the European Research Council, the European Molecular Biology Organization Young Investigators Programme, the Lister Institute of Preventative Medicine, and the Medical Research Council.
The study was published in the peer-reviewed Proceedings of the National Academy of Sciences (PNAS) journal.
Some sections of the UK media have produced muddled reporting, and the average reader would be forgiven for being very confused.
For example, BBC News reported that "Viruses 'more dangerous in the morning'," while the Daily Mail tries to explain, "Why you're more likely to catch a cold in the morning", but the New Scientist tells us "Herpes infections are worse if contracted at the end of the day".
The apparent root of all this confusion is that there is no mention in the media that we cannot be sure how these findings will transfer to humans.
Mice are nocturnal animals, so the timing of their body clocks is entirely different from those of humans – at least, those who work nine to five.
What kind of research was this?
This study in mice aimed to establish whether the time of day a virus is caught affects the spread.
The researchers hypothesised that the internal body clock, which is constantly switching functions on or off, may have an impact on the spread of a virus. This is because when a virus enters the body, it uses our cells to spread.
While findings from animal studies are useful to see how biological processes may work in humans, our cells are not identical to those in mice.
This means the findings seen in this research may not be directly transferrable to humans.
What did the research involve?
The researchers infected mice with either the influenza or herpes virus at the start of the day, the beginning of their resting phase, or at the start of their active phase, 10 hours into their day.
Two genetically different types of mice were used in the experiment – some with the gene that controls the body clock, and some with it knocked out.
After being given the virus, the mice lived in an environment where they spent 12 hours in daylight and 12 hours in darkness.
After six days, cells from the mice were analysed to assess the amount of virus and the level of spread.
What were the basic results?
When mice were given the virus at the start of the day – when nocturnal animals are starting their daily resting phase – the replication of the virus was 10 times greater than in mice given the virus at the start of their active phase.
When experimenting on mice without the body clock gene, researchers found high levels of the virus regardless of the time of day the mice were infected.
How did the researchers interpret the results?
The researchers concluded that their work has shown viruses exploit the clockwork for their own gain, and the body clock plays a part in controlling the spread of a virus.
Conclusion
This novel animal study aimed to establish whether the time of day when a virus is caught affects its spread.
The findings do seem to suggest – in mice at least – being infected at the start of the resting period led to greater viral replication than being infected during the active part of the day.
The researchers confirmed this by demonstrating that mice without the body clock gene showed a high level of the virus regardless of the time of day they had been infected.
Circadian rhythms are biological cycles in the body related to the time of day. They are sometimes referred to as the body clock, or as the body's individual biological timing.
The body's cells have their own clocks, which interact with each other and are controlled by this master 24-hour clock in the brain.
It is this effect on the cells that the researchers feel is responsible for the differences in viral spread.
These findings may cause concern for those with a disrupted daily pattern, such as shift workers.
Taking a great leap, you could think, for example, that if night shift workers go out to work and catch a virus, they are catching it at the start of their resting period, so it will replicate more.
But there are a number of cautions to this thinking:
- Human cells are not identical to those in mice, so we don't know that the findings seen in this research directly apply to humans.
- Even if the processes are similar, the body clock likely shifts in people who regularly work nights or shift patterns, so their body is geared up to being active at this time.
- Even if there is greater viral replication, we don't know whether the differences in the extent of the replication are enough to cause greater illness or more debilitating symptoms in the individual.
There are a number of easy steps that can be taken to reduce your risk of catching or spreading a virus.
These include practising good hygiene by always washing your hands, keeping surfaces like keyboards and telephones clean, and, if you have a virus, making sure you use tissues to cover your mouth and nose if you cough or sneeze.
The case could be made that, especially in the event of a future flu pandemic, shift workers should be added to the list of those thought to be especially vulnerable to the effects of an infection.
