When Your Clock and the Sun Disagree
Jet lag occurs when rapid travel across time zones desynchronizes the internal circadian clock from the local environment. Your SCN still runs on departure-city time while the sun, meals, and social cues demand arrival-city time. The result: insomnia at night, fatigue during the day, digestive problems, and impaired cognitive performance that can last for days.
The Asymmetry of Direction
Eastward travel is consistently harder than westward. The human clock's intrinsic period of ~24.2 hours makes delaying (staying up later, as needed for westward travel) easier than advancing (going to bed earlier, as needed for eastward travel). This asymmetry is so reliable that mathematical models predict recovery time from direction alone, with eastward trips taking 30–50% longer.
Light as Medicine
Timed light exposure is the most powerful tool for accelerating re-entrainment. Morning light at the destination advances the clock (helpful after eastward travel), while evening light delays it (helpful after westward travel). For shifts larger than 8 hours, the optimal light window reverses — getting it wrong can push the clock in the wrong direction and prolong jet lag.
Individual Differences
Age, chronotype, and prior sleep debt all affect jet lag severity. Younger individuals with flexible schedules adapt faster. Night owls handle westward travel better; morning larks handle eastward travel better. Frequent travelers develop behavioral strategies but don't develop physiological tolerance — the clock always needs time to shift.