Why your body clock cannot simply reset
Jet lag is a mismatch between the time on the clock and the timing of the body’s daily rhythms. The brain’s main clock helps coordinate sleep and wakefulness, while also influencing processes such as body temperature and hormone release. A time-zone jump changes the local schedule instantly; those internal rhythms do not move in step.
The clock resets mainly in response to cues from the environment, especially light. Morning light tends to shift the body clock earlier, while evening light tends to shift it later. That makes the direction of travel matter: adjusting after flying east, where local time is ahead, can require a different light-and-dark pattern than adjusting after flying west.
Why the effects linger
Until the rhythms catch up, they can send conflicting signals. You may feel sleepy when you need to be alert, or wake in the middle of the local night because your body still treats it as daytime. Meals, work, and social plans follow the destination’s clock, but the body is still running partly on the departure schedule.
The shift happens gradually rather than as a single reset. Sleep may begin to improve before every internal rhythm has fully aligned, so feeling better does not necessarily mean the adjustment is complete. A long trip can therefore leave several days in which the local morning arrives while the body is still prepared for night.
