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Your body's internal clock naturally runs a little longer than 24 hours, which makes it easier to delay your schedule (stay up later, as when flying west) than to advance it (fall asleep earlier, as when flying east). That's why the same number of time zones crossed eastward typically causes more jet lag than crossing them westward.
As a rough rule of thumb, plan on about one day of adjustment per time zone crossed eastward, and roughly two-thirds of a day per time zone crossed westward — though this varies by individual, sleep habits, and how much daylight exposure you get on arrival.