New research indicates our immunity to infection might be very closely tied to our daily circadian rhythms — more commonly known as our body clocks.

Researchers already know that disease-fighting blood cells ebb and flow with our internal clocks. In fact, levels of circulating T-cells peak at night and then gradually subside, which could explain why we get so tired when we’re sick — sleeping may actually help us fight off infection.

Now a new study with mice at Yale University, published in the journal Immunity, showed that even the success of vaccinations could depend on these circadian fluctuations.

Researchers found that sleep cycles influenced levels of an immune protein called Toll-like receptor 9, or TLR9, which has a daily peak and then drops off. When mice were exposed to bacteria at the low point of TLR9 activity, they developed severe sepsis and died much sooner than those exposed when TLR9 was at its peak. In addition, mice vaccinated near TLR9′s daily high point had stronger immune responses than those vaccinated when it was low.

The scientists say it’s possible these fluctuations evolved to help protect us against diseases like malaria, which is carried by mosquitoes that typically have daily feeding patterns. It could also explain why research has shown jet lag — which throws off our natural body clocks — can have such an effect on our immune systems.