Fig. 2: The Phase Response Curve (PRC).
From: Beyond vision: effects of light on the circadian clock and mood-related behaviours
A Brief light exposure during the early subjective night (blue arrow, left panel) will delay the onset of the subsequent activity cycle, resulting in a phase delay of the clock characterized by a negative Δφ (phase angle difference of the two red lines of activity onset). By contrast, light applied in the late portion of the subjective night (blue arrow, right panel) induces phase advances (positive Δφ). This change in phase can be plotted as a function of time when light is applied, resulting in the phase response curve (B), which serves as a read-out for how light entrains (resets) the clock.
