It is known that Chlamydia pathogenesis causes complications such as pelvic inflammatory disease and tubal factor infertility. In this study, we aim to understand how host circadian rhythms impact the outcome of Chlamydia infection by determining how the time of day of infection influences its pathogenesis. We hypothesize that mice infected in the morning will experience increased pathogenesis compared to those infected in the evening. Mice were divided into two groups: mice were put under normal 12/12 light and dark cycle, and under constant light. Mice were infected intravaginally with Chlamydia muridarum at 10:00 am (morning infected) and 10:00 pm (evening infected). Vaginal swabs were taken every three days for 27 day and blood samples and vaginal washes were collected once a week. Following infection, half of the mice in each group were sacrificed, and the reproductive tract removed for determination of pathological changes. The remaining mice in the normal light group were mated to determine their fertility. Results from this study showed that mice housed under normal light that were infected in the morning shed significantly more Chlamydia than mice infected in the evening. This correlated with the increased pathology noticed in mice infected in the morning compared to mice that were infected in the evening. In addition, mice infected in the morning were less fertile compared to mice infected in the evening. However, mice housed under constant light had increased Chlamydial infectivity, which was not significantly different based on the time of infection; the pathology was also the same for morning and evening infected mice. This study is the first to show an association between Chlamydia infection and the time of day of infection. Results from this study suggests a role for host circadian rhythms in the pathogenesis caused by Chlamydia. Understanding the role of circadian rhythms will provide us with new and novel means of delineating Chlamydial pathogenesis.