Although circadian rhythms are pervasive in all life forms, the topic has only recently garnered prominent scientific attention. The discovery of the ipRGC in the early years of this century, identifying the retina as providing the primary signal to the circadian system, has impelled new scientific interest in chronobiology. In addition to early attention to common jet lag symptoms and seasonal affective disorder (SAD), researchers have been identifying ever more health consequences of disrupted circadian rhythms, especially related to sleep quality. The public health implications are enormous. These physiological processes are complex, with many feedback loops, and are not yet thoroughly understood. Furthermore, we do not yet fully understand the what optimum, healthy exposure to sunlight or daylight looks like in the modern world. Until we do, prescriptions for artificial lighting solutions inside of buildings are premature. In the meantime, we can be certain that illumination from daylight is natural and always locally appropriate. Our experience of a hunger for daylight may be best described as a natural appetite.