Eastward travel needs a phase advance (clock moves earlier). Westward needs a phase delay. Low-dose melatonin in the late afternoon advances; melatonin in the morning delays. Bright light is the strongest zeitgeber: morning light advances, evening light delays. Crucially, light at the wrong time can undo the shift, so the plan tells you both when to seek light and when to avoid it. Body adjustment rate is roughly 1 h/day eastward and 1.5 h/day westward — long shifts can't fully complete during short trips. Shifts > 12 h are treated as the shorter direction (e.g., 14 h east ≈ 10 h west). With multi-leg itineraries, the destination is auto-detected as the airport where you spend the longest continuous stay; layovers are ignored for circadian planning but listed in the travel-day card.