It has been demonstrated that from the early stages of postnatal life up to adult age, gradual development of circadian amplitudes invariably takes place which may lead up to a complete absence of the diurnal rhythm in senile organisms. These changes are observed at various levels of organization of homeostatic systems (from cellular to organismic ones). A discussion is made of a possibility of evaluation of the level of adaptability and reliability of biological systems, as well as of their functional optimum via the analysis of circadian organization in ontogenesis, including gerontological problems (differentiation into age periods, biological age).