In tissues such as bone marrow, intestinal mucosa, or regenerating liver, the daily rhythm of cell division is controlled by the cell’s circadian clock. Determining how this clock organizes important processes such as cell division, apoptosis, and DNA damage repair is key to understanding the links between circadian dysfunction and malignant cell proliferation. We show that in proliferating mouse fibroblasts there is more than one way in which the clock and cell cycle synchronize their oscillations and that one of them is the biological equivalent of the phase locking first discovered by Huygens in the 17th century when he coupled two clocks together. When phase-locked two coupled oscillators have a fixed relative phase and oscillate with a common frequency.