A typical proliferating human cell divides on average every 24 h. This division timing allows cells to synchronize with other physiological processes and with the environment. The circadian clock, which orchestrates daily rhythms, directly regulates the cell division cycle and is a major synchronizing factor. There is, however, no evidence that the circadian clock is able to entrain the cell cycle to a 24 h period. We show here, using a computational model for the cell cycle, that cells under circadian control that have an interdivision time close to multiples of 24 h proliferate faster. Moreover, growth of cell populations with a markedly different cell cycle time is impaired. We propose that this resonance effect in cell proliferation has a role to play in efficient normal cell proliferation and suppression of tumor growth.