(left) The circadian clock operates as a network where the master pacemaker and slave oscillator are organized in a hierarchical manner. Although they generate rhythms via an identical transcriptional-translational feedback loop (TTFL) of PER proteins, the generated PER rhythms are different between them. (right) By using a mathematical model describing the TTFL of the Drosophila circadian clock (top), we identified the differences between master and slave clock neurons (bottom).