Amalfitano had some rather idiosyncratic ideas about jet lag. They weren't consistent, so it might be an exaggeration to call them ideas. They were feelings. Make-believe ideas. As if he were looking out the window and forcing himself to see an extraterrestrial landscape He believed (or rather like to think he believed) that when a person was in Barcelona, the people living and present in Buenos Aires and Mexico City didn't exist. The time difference only masked their nonexistence. And so if you suddenly traveled to cities that, according to this theory, didn't exist or hadn't yet had time to put themselves together, the result was the phenomenon known as jet lag, which arose not from your exhaustion but from the exhaustion of the people who would still have been asleep if you hadn't traveled. This was something he'd probably read in some science fiction novel or story and that he'd forgotten having read.
2666 by Roberto Bolaño, page 189.
Update: From this post and its comments, it seems likely that the "science fiction novel or story and that he'd forgotten having read" was William Gibson's Pattern Recognition or Brian Fawcett's Soul Walker. From Pattern Recognition:
She knows, now, absolutely, hearing the white noise that is London, that Damien's theory of jet lag is correct: that her mortal soul is leagues behind her, being reeled in on some ghostly umbilical down the vanished wake of the plane that brought her here, hundreds of thousands of feet above the Atlantic. Souls can't move that quickly, and are left behind, and must be awaited, upon arrival, like lost luggage.
(thx, thessaly & michael)