Light modifies the biology and physiology of mammals, including humans. The cellular biology of both the immune system, as well as, the cells comprising tissues and organs (e.g., kidney, liver) are under the regulation of light and exhibit circadian rhythms. Studies show that the severity of organ injury varies with the time of the day, the duration of the day, and the season. This variation is due to the biology of circadian rhythms, and light is the principle environmental cue entraining circadian biology. More recent data suggest that modulating the spectrum of light to which an organism is exposed may therapeutically modulate the cellular response to stress or injury. Specifically, exposure to a short (24 hour) photoperiod of high illuminance, blue spectrum light attenuated liver and kidney injury when animals were subjected to ischemia/reperfusion (I/R), a model in which the blood flow to an organ is temporarily reduced and then restored. A predominant cause of organ injury in this model is the misdirected and exacerbated inflammation of a type of immune cell called the neutrophil. However, blue light inhibited neutrophil infiltration into the ischemic kidney and liver, and thereby reduced inflammation and neutrophil-mediated organ injury.