Circadian rhythm is the body's internal clock which regulates the sleep/wake cycle and controls the expression of specific genes. Influenza A virus is a leading cause of morbidity and mortality that primarily affects the lungs and is responsible for more than 200,000 hospitalizations and 36,000 deaths a year. In this lab, genetically modified mice were infected with Influenza A virus and the damage to the lungs were observed. We specifically assessed if the role of Natural Killer (NK) cells in circadian regulation of lung inflammation is mediated by anti-viral or host tolerance pathways. NK cells are lymphocyte and component of innate immune system composing of 5-10% of resident immune cells. In previous experiments, the outcome of influenza A infection, viral burden and pulmonary infection were demonstrated to be influenced by circadian rhythm. Furthermore, circadian regulation was shown to be mediated by worse injury (histology) and not viral burden. By repeating previous experiments and depleting NK cells through the NK1.1 antibody, we found that the temporal patterning of the inflammatory response was mediated not by control of viral burden, but by worse lung histology. Further elucidating circadian regulation in pulmonary inflammation will be helpful to strategize timing of drugs/vaccines. In addition, it may provide the opportunity to identify new therapeutic targets to improve host tolerance.