Why can jet lag upset your stomach even after a decent sleep on the plane?

Answer: Digestive rhythms misalign

Spoiled airline food — Bad food can cause stomach trouble, but jet lag does not require food poisoning. Mayo lists stomach and bowel problems among jet-lag effects, and Frontiers ties poor gastrointestinal function to circadian mismatch after time-zone travel. The useful distinction is illness versus timing: the body can be healthy but scheduled wrong.

Digestive rhythms misalign ✓ — Jet lag is a body-wide timing problem, not just a sleep problem. Digestion, hunger, and bowel timing can remain out of step with local meals even after you got some sleep. This helps explain why stomach symptoms can survive a decent in-flight nap: sleep pressure improved, but internal schedules still disagree.

Cabin pressure alone — Cabin pressure can contribute to travel discomfort, but it is too narrow to explain destination-time digestive rhythm problems. The more interesting mechanism is internal desynchrony: organ clocks and meal timing can be out of phase with local time. Jet lag feels diffuse because the body is not one clock.