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Phase response curve

The map of how far a pulse of light will shift a clock, depending on when it arrives relative to what the clock currently believes the time to be. Its shape is not intuitive. Light landing in the early subjective night pushes the clock *later*; light in the late subjective night, just before the internal dawn, pulls it *earlier*; and light in the middle of the subjective day does almost nothing - a stretch called the dead zone, where the clock has already had its fill of daylight and one more photon carries no information. That three-part shape is exactly what makes daily resetting work.