The Traveler's Dilemma
Jet lag is the visceral experience of circadian misalignment — your internal clock says 3 AM but the local clock reads noon. After crossing multiple time zones, every circadian-controlled process is temporarily out of sync with the new environment: sleep, alertness, digestion, hormone secretion, body temperature, and immune function. The result is a constellation of symptoms — insomnia, daytime fatigue, cognitive impairment, GI distress — that typically takes one day per time zone crossed to resolve fully.
Asymmetric Recovery
A fundamental asymmetry governs jet lag: westward travel is consistently easier than eastward. This stems from the human circadian pacemaker's intrinsic period of ~24.2 hours. Because the clock naturally drifts later each day, delaying it (westward) requires less corrective light input than advancing it (eastward). Mathematical models predict that westward re-entrainment proceeds at ~1.3 h/day versus ~0.9 h/day eastward — closely matching epidemiological data from flight crews and frequent travelers.
Light as Medicine
Strategic light exposure is the most powerful tool for accelerating jet lag recovery. The phase response curve dictates optimal timing: for eastward travel, bright outdoor light in the destination's morning advances the clock; for westward travel, bright evening light delays it. Crucially, light at the wrong time can shift the clock in the wrong direction (antidromic re-entrainment), especially for large time zone shifts. This simulator models daily phase shifts based on light timing and intensity, showing the recovery trajectory day by day.
Beyond Light: Melatonin and Behavior
Melatonin supplements provide a complementary phase-shifting signal. Taken at the destination bedtime, exogenous melatonin (0.5–5 mg) advances the clock by 30–90 minutes per day, working synergistically with morning light. Behavioral strategies also matter: maintaining regular meal times, exercising in the afternoon, and avoiding naps longer than 20 minutes all support faster re-entrainment. This simulator integrates light exposure, melatonin, and chronotype to predict personalized recovery trajectories.