FIGURE 1.
(A) An asymptotically stable limit cycle of a simple two-state Drosophila circadian model. The phase φ is defined as the time distance between the reference and the current state on the limit cycle (modulo the period). (B) Two trajectories in a limit cycle with a phase difference of Δφ; the solid trajectory leads the dashed, or vice versa, the dashed trajectory lags the solid.