The clock you brought with you
Your circadian system runs on a ~24.2-hour internal cycle, set daily by light. Cross eight zones and the clock still reads home: melatonin arrives at 3 pm local, body temperature dips at noon, and the "afternoon slump" hits at what your body insists is 3 am. Jet lag is not tiredness — it is the entire physiology operating on the wrong clock face.
The asymmetry you feel
The clock's natural period is slightly LONGER than 24 hours, which makes delaying (westward) the downhill direction and advancing (eastward) the uphill one. London→LA recovers at ~0.6 days/zone; LA→London at ~1.0. Every traveler's lived experience of "the way home was easier" is this asymmetry, measurable and repeatable.
The lever hierarchy
Light is the master signal (the clock evolved to follow it) — timed exposure shifts it 1–2 hours per day. Meals and activity are secondary signals. Melatonin and caffeine are tactical supports. Travelers who go straight to the supplements while spending day one indoors have chosen the weakest lever first — the light schedule is the mechanism, everything else is decoration.