According to new findings, viruses use changes in their hosts’ physiology to maximize infection efforts, including playing off of circadian rhythms and internal clocks to take advantages of weak points.

This means time of day or abnormal sleep schedules could impact a person’s (or other animal’s) susceptibility for infection. The findings might explain why viral infections tend to increase at the population during certain seasons or during periods of sleep-wake schedule disruption.

The researchers explained that while circadian rhythms influence organisms as a whole, specific cells also have their own internal clocks. Viruses take over specific cells, meaning they might use those specific internal cell clocks to their own advantages.

The study, published in the journal Proceedings of the National Academy of Sciences of the United States of America, focused on the interaction between viruses and mice. They infected the mice with a strain of rodent herpes and found that the mice infected just as their resting phase began saw 10 times higher viral replication than mice injected with the virus just as their active phase of the day began. But another type of mice with a mutated/non-existent internal clock function infected with the same virus at the same times showed no difference in level of infection. They also found that circadian rhythms also impacted the infection cycle of herpes simplex virus 1.

They concluded the varying infection times and severities weren’t just related to the circadian-regulated portions of the mice’s immune systems either—it was the functions of the infected cells themselves that were affecting the virus’ ability or inability to replicate. And they found that during the initial infection period, at the times when the immune system was at its strongest, the viral replication would slow down, possibly to avoid detection by the immune system that would provoke an attack on the virus.

Disrupting the cell’s internal clocks allowed for the faster, stronger spread of influenza A in the mice. The researchers speculated on the mechanism by which this is all taking place:

“For pathogens such as Plasmodia, which cause malaria, synchronizing their replication cycle with host circadian rhythms contributes to their success. Likewise, we speculate that coevolution of viruses with their host clocks enables them to capitalize on the predictability of daily rhythms driven by cell autonomous molecular clocks,” the authors wrote.

Because the circadian rhythms are not just related to daily environmental changes but also seasonal changes, viruses could also be taking advantage of those.

“We speculate that this may contribute to viral dissemination at the population level because many viruses, including influenza, cause infection more commonly in the winter,” according to the study authors.

This information could have useful applications for humans trying to avoid viral infections, not just the lab mice. The study researchers called these new findings evidence that shift workers, whose daily schedules might be disrupted from their natural inclinations, could see disproportionate benefits from flu vaccines. And that the time of day flu vaccines are distributed among the elderly could make a difference in their bodies’ abilities to use the vaccines to mount effective immune defenses.