Yin-yang concepts of traditional Chinese medicine (TCM) for TCM objects such as whole body, five zangs or six fus are widely used to discuss cellular processes. In this paper, the concept of the degree of difficulty (DD) of a process was put forward to redefine yin and yang and extend TCM yin-yang model to DD yin-yang model. It was shown that healthy cells are in DD yin ping yang mi so that there is no photobiomodulation, and there is photobiomodulation on non-healthy cells until the cells become healthy so that photobiomodulation can be called cellular rehabilitation.