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Why does jet lag occur after traveling across time zones?

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Jet lag is a form of physiological desynchronization that occurs when rapid travel across multiple time zones disrupts the body's internal circadian rhythms.21

These rhythms, which regulate essential functions like sleep, body temperature, and hormone secretion, are naturally synchronized to a 24-hour cycle by environmental cues known as zeitgebers—most notably the alternation of light and dark. The suprachiasmatic nucleus in the brain acts as the primary regulator of these cycles, using light signals from the eyes to control the production of melatonin by the pineal gland.21

When you cross time zones, your internal clock remains set to your point of origin, causing your melatonin production and other physiological patterns to fall "out of sync" with the local light-dark cycle at your destination. Because these various biological processes do not readjust at the same rate, you experience symptoms such as fatigue, sleep disturbances, and disorientation until your internal rhythm generator can fully synchronize with the new local time.21