The endogenous circadian timing system has evolved to synchronize an organism to periodically recurring environmental conditions. Those external time cues are called Zeitgebers. When entrained by a Zeitgeber, the intrinsic oscillator adopts a fixed phase relation Formula: see text to the Zeitgeber. Here, we systematically study how the phase of entrainment depends on clock and Zeitgeber properties. We combine numerical simulations of amplitude-phase models with predictions from analytically tractable models. In this way we derive relations between the phase of entrainment Formula: see text to the mismatch between the endogenous and Zeitgeber period, the Zeitgeber strength, and the range of entrainment. A core result is the "180 rule" asserting that the phase Formula: see text varies over a range of about 180 within the entrainment range. The 180 rule implies that clocks with a narrow entrainment range ("strong oscillators") exhibit quite flexible entrainment phases. We argue that this high sensitivity of the entrainment phase contributes to the wide range of human chronotypes.
Granada, A. E., Bordyugov, G., Kramer, A., & Herzel, H. (2013). Human Chronotypes from a Theoretical Perspective. PLoS ONE, 8(3). https://doi.org/10.1371/journal.pone.0059464