In most eukaryotic organisms, networks of cell cycle and circadian rhythms coexist and work coordinately to create optimal conditions for cells to grow and adapt to the surrounding environment. Cell cycle regulatory mechanisms include multiple checkpoints for controlled growth and cell divisions. The period of this oscillation, however, varies with external conditions such as nutrient and temperature. The cell cycle machinery is optimized for growth and division, but not for time keeping. Circadian rhythms keep track of time and provide temporal regulations in most eukaryotic organisms with a period of about 24 h. In contrast to the period of the cell cycle, the period of circadian rhythms is relatively insensitive to external conditions such as nutrient and temperature. Cell cycle and circadian rhythms are coupled despite of their apparent disparate functions. The circadian gated cell division cycles are observed in various organisms from cyanobacteria to mammals. However, the implications of this coupling on the physiology of an organism are unknown. We use a mathematical model to study interactions between the cell cycle and the circadian clock and their implications in cell cycle regulations.