The temporal order of physiological functions such as sleep/wakefulness is regulated by the circadian clock. This intrinsic clock starts ticking in the embryo and matures during development, with attenuation of the clock function in the elderly, illustrated by attenuation of synchrony, entrainment, and outputs of cellular circadian rhythms in the SCN. This age-related diminution can contribute to the emergence of diseases, such as sleep disorders, infertility, diabetes, mental disorders etc. Over the course of our lives, a variety of internal and external factors are under the influence of the circadian clock. This inherent developmental plasticity of the circadian system is critical for the establishment of normal bodily functions to adapt to the changing environment on earth. In mammals, the central circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Circadian rhythms in the SCN are observed during the embryonic period, and input and output pathways from the SCN are formed after birth. During postnatal development, functions of neurotransmitters in the SCN are changed, which have an influence on special temporal patterns of circadian rhythms in the SCN and outputs signals to the outside of the SCN. During postnatal development properties of circadian rhythms in the SCN and its inputs and outputs can be modulated by environmental light conditions. Recent optical manipulation and imaging techniques have revealed that cell type specific functions for circadian rhythms. However, it remains unclear which cell types or networks in the brain are modulated during postnatal development, and how these cells regulate circadian rhythms depending on postnatal period.