It's the Zones, not the Flight Time

While long-haul flights are certainly exhausting across the board, jet lag is not caused by the number of hours clocked on a plane—but by changing time zones. “Rapid travel between time zones temporarily disrupts this biological clock," says Chris Colwell, a professor in psychiatry at the UCLA Medical School. “Our system does readjust, but this takes time. It can take a couple of weeks to adjust to the new time in Europe or Asia.” A traveler flying from New York to Madrid, for example, will change time zones six times along the way.