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The innate and adaptive immune response are regulated by biological clocks, and circulating lymphocytes are lowest at sunrise. Accordingly, severity of disease in mouse models is highly dependent on the time of day of viral infection. Here, we explore whether circadian immunity contributes significantly to seasonality of respiratory viruses, including influenza and SARS-CoV-2. Susceptibility-Infection-Recovery-Susceptibility (SIRS) models of influenza and SIRS-derived models of COVID-19 suggest that local sunrise time is a better predictor of the basic reproductive number (R 0) than climate, even when day length is taken into account. Moreover, these models predict a window of susceptibility when local sunrise time corresponds to the morning commute and contact rate is expected to be high. Counterfactual modeling suggests that retaining daylight savings time in the fall would reduce the length of this window …