Research suggests that your body’s circadian rhythms—as well as some other factors—can exacerbate your symptoms after sundown, reports TIME.

Along with regulating your sleep, your body’s circadian clocks help manage your immune system, says Michael Smolensky, a biological rhythm researcher and adjunct professor of biomedical engineering at the University of Texas. “When the immune system is activated”—like when you’re sick with the common cold—“its infection-fighting cells release a variety of chemicals, some of which induce inflammation in the infected tissues,” he explains.