Function-specific homeostasis (FSH) is a negative-feedback response of a biosystem to maintain the function-specific conditions inside the biosystem so that the function is perfectly performed. Low level light (LLL) is so defined that it can not modulate the function in its FSH,but it can modulate the function far from its FSH. FSH quality is represented by function complexity and FSH stability. The FSH stability is maintained by longevity factors which mediate longevity extension. The function complexity and FSH stability are specific with each other so that there is FSH-specific longevity factor activity for each one. Among the longevity factors,sirtuins (SIRTs) are the most famous family. There is FSH-specific SIRT activities (FSSA). The FSSA increase as the FSH quality goes up. For the cells which SIRT activities are lower than FSSA,LLL may increase SIRT activities until the SIRT activities arrive at FSSA,respectively. For the cells which SIRT activities are higher than FSSA,respectively,but they can be back in FSH,LLL may increase the SIRT activities of the other cells,which promotes the rehabilitation of the cells so that they may be back in FSH. For the cells which SIRT activities are much higher than FSSA,respectively,LLL may promote the establishment of a new FSH of higher FSSA. In summary,the cellular optical rehabilitation may be mediated by SIRTs.