Jet lag is caused by rapid travel across time zones that temporarily desynchronizes your internal biological clock from the local day–night cycle at your destination. According to the CDC Yellow Book (2026), that mismatch—not ordinary travel fatigue—drives sleep disturbances, daytime sleepiness, cognitive fog, malaise, and sometimes gastrointestinal symptoms.
The more time zones you cross, the larger the shift your circadian system must make. Eastward travel usually feels harder than westward travel because advancing the body clock (sleeping and waking earlier) is more difficult for most people than delaying it. Until light cues and sleep schedules re-entrain the clock, destination “wall time” and your body’s preferred sleep window remain out of sync.