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Why does the body take several days to adjust to a new time zone?

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The body requires several days to adjust to a new time zone because its internal physiological processes, known as circadian rhythms, are governed by a complex, synchronized system that cannot reset instantly.12

These rhythms control essential functions—including sleep and wakefulness, body temperature, hormone secretion, and digestion—which are driven by light-dark cycles detected by the eye. When you travel across time zones, your internal clock, regulated by the suprachiasmatic nucleus (SCN) in the brain, becomes desynchronized from the local environment.12

The SCN manages the production of melatonin by the pineal gland; however, the brain also utilizes specific proteins, such as SIK1, that act as a "brake" on the body's ability to immediately adapt to new light-dark patterns. Because these various physiological processes do not return to synchrony at the same rate, the body experiences jet lag, typically requiring one day of adjustment for every hour of time difference.12