Biological rhythms occur at different levels in the organism. In single cells, the cell
division cycle shows rhythmicity in the way its molecular regulators, the cyclin dependant
kinases (CDKs), modulate their activity periodically to ensure a healthy progression. In
tissues, cell proliferation is driven by the circadian clock, which modulates the
progression through the cell cycle along the day. The circadian clock shows endogenous
rhythmicity through a robust network of transcription-translation feedback loops that
create sustained oscillations. Rhythmicity is preserved in cell populations by the
coordination of the clocks among cells, through rhythmic synchronization signals. Here we
discuss mechanisms for generating rhythmic activities in cell populations by reviewing
some of the mathematical models that deal with them. We discuss the implication of
biological rhythms for tissue growth and the possible application to chronomodulated
cancer treatments.