Slide the light pulse across the circadian cycle (CT = circadian time, CT12 = activity onset/dusk, CT24/CT0 = dawn) and see the resulting phase shift.
Resulting phase shift: —
Subjective day (CT0–12) is the "dead zone" — light has little effect. Early subjective night (~CT12–18) produces delays; late subjective night (~CT18–24) produces advances — the same logic behind eastward vs. westward jet lag.