Biological clocks are intrinsic time-keeping systems that regulate behavior and physiological functions in most living organisms. Recent works in this area have addressed possible molecular links between the endogenous circadian clock and cell cycle regulation. In this review, by addressing how circadian clocks can interfere with the cell cycle and how the disruption of the circadian rhythm may cause defects in regulation of cell proliferation, we highlight this potential connection between circadian rhythm and cell cycle. We also discuss how the acquisition of recent data in circadian clock mechanism may help chronotherapy, which takes into account the biological time to improve cancer treatments, and may open new therapeutic avenues for treating circadian-related diseases.