There are plenty of old wives’ tales about what gets you sick — the myth about going out in the cold, for example — but new research in mice suggests that the time of day at which an infection starts up could play a role in how sick you get. Researchers found that, in mice that were infected with a virus in the morning, the virus replicated within the cells of those mice much more than it did in the mice that were infected with the same virus later in the day. The difference may be due to the mice’s circadian rhythm, or biological clock, according to the study published on August 15th in the journal Proceedings of the National Academy of Sciences. In addition to influencing when we go to sleep and wake up, circadian rhythm helps regulate some aspects of the immune system, the researchers wrote. Because the study was in mice, not humans, it’s not clear if the results also apply to humans. Indeed, the immune system undergoes repair during the resting phase of the circadian rhythm, and is “primed for pathogen attack at the onset of the active phase,” according to the study.

In the experiment, the researchers infected normal mice with the herpes virus, and then measured the virus’s replication in the cells of the animals. The mice were on a 24-hour schedule, with 12 hours of daylight and 12 of darkness. They found that the viral replication was 10 times greater in the mice that were infected with the virus at “sunrise” than in those that were infected 10 hours into the day. (In mice, which are nocturnal, sunrise is when they begin their resting phase.) Ten hours into the day marks the start of their active phase, according to the study. When the researchers repeated the experiment with mice that were engineered to lack a gene linked to their circadian rhythm, they found that viral replication was high no matter what time of day the mice were infected. The researchers also did an experiment using mouse cell cultures in the lab. In addition to observing greater viral replication in cells that lacked a circadian rhythm, the researchers found that the herpes virus was able to alter the cell’s biological clock mechanism, making the cells more vulnerable to infection.