Unexpectedly, Americans discovered that rapid transit across several time-zones took a physical toll on them. Air travelers often complained of physical and mental exhaustion for days after a long flight. In 1958 researchers at the University of Minnesota attributed this malady to the temporary disruption of the body's "circadian rhythms" — fluctuations in heart rate, temperature, and other bodily processes that correspond to the cycle of day and night. Since a plane flight from one time zone to another distorted the length of its passengers' day, it also threw their metabolisms out of whack. During the 1960s this phenomenon came to be called "jet lag."

Source:

"Travels of the Jet-Age Traveler," Popular Science, 172 (May 1958): 93.