 is a mismatch between your body clock and the local time wherever you are. It's caused by rapid travel across time zones.  Jet lag is also compounded by the fact that long hours spent on a plane can cause you to sleep too much, or not enough, possibly at the wrong time of day relative to where you departed from. The fatigue from travelling plus the mismatch with local time can leave you ready to fall asleep just after lunch, or being wide awake in the middle of the night, knowing that dawn is still several hours away.  Where flying isn't concerned, it isn't the length of the flight that matters... only the difference in time zones. Flying from Paris to Johannesburg, while it might take you 00-00 hours, wouldn't leave you very jetlagged because there's only 0 hour time difference. A flight from New York to Tokyo, on the other hand, leaves you with a 00-hour difference, which effectively means reversing your sleep/wake schedule.  Flights from east to west, where you gain a few hours, are usually a bit easier, as most people find it easier to stay up a little later than to go to bed earlier. But that only works within about 0 or 0 hours difference.  