The science
Your clock travels at its own pace
When you land, your body clock may still be closer to the time you left behind.
Where you are in your daily rhythm
Your body clock has its own timing for sleep and wakefulness. Scientists call your position in that daily rhythm your circadian phase.
The temperature low point
Your core body temperature follows a daily rhythm, with a low point usually during your biological night. Its timing varies between people, so it is not a fixed hour on the clock.
Why local time can feel out of step
After a time-zone change, local bedtime may arrive while your body is still ready to be awake. Being tired from the journey and having your clock on a different time are separate things.
Earlier or later
Moving your clock earlier is called an advance; moving it later is a delay. Eastward travel often calls for an earlier clock and westward travel for a later one, but journeys east across many time zones can be more complicated.
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