After crossing several time zones a traveler feels exhausted during the local day and wide awake at night. That disruption is jet lag. It results from the body's internal clock falling out of sync with the destination's light-dark cycle.

Symptoms include sleep disturbance, daytime fatigue, irritability, and reduced concentration. Eastward travel generally causes stronger effects than westward travel because shortening the day is harder to adapt to. Recovery typically takes roughly one day per time zone crossed, though individual variation is large.

Strategies such as adjusting sleep schedules before departure, seeking morning light after eastward flights, staying hydrated, and using short strategic naps can reduce severity. Melatonin is sometimes used under guidance. Important meetings or performances are best scheduled after a recovery buffer when possible.

Jet lag reminds travelers that the body keeps its own time regardless of the airport clock.