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It's been only in the past 150 years that we've been able to jump across time zones, and we haven't evolved a way to adapt yet. Eastward travel is more difficult than westward because our body clock prefers a twenty-five-hour day. Therefore, we can more easily stay awake an extra hour than fall asleep an hour early. Westward travel finds us having to delay our bedtime, which is not so difficult to do. Eastward travel finds us arriving in a city where it's bedtime and we're not yet tired. Traveling east is difficult even for people who do it all the time. One study of nineteen Major League Baseball teams found a significant effect: Teams that had just traveled eastward gave up more than one run on average in every game.

Aligning your body clock to the new environment requires a phase shift. It takes one day per time zone to shift. Advance or retard your body clock as many days before your trip as the number of time zones you'll be crossing. Before traveling east, get into sunlight early in the day. Before traveling west, avoid sunlight early by keeping the curtains drawn, and instead expose yourself to bright light in the evening, to simulate what would be late afternoon sun in your destination.