Jet Lag Calculator:
your day-by-day adjustment plan.

Field guide

The science of beating jet lag.

Jet lag isn't fatigue from a long flight — it's the mismatch between your body's internal clock and the clock on the wall at your destination. The fix is to resynchronize the clock, and the levers that work are well studied: timed light exposure, scheduled sleep, melatonin, and caffeine timing.

Your master clock and the CBT minimum

A cluster of neurons in your hypothalamus — the suprachiasmatic nucleus (SCN) — runs on a roughly 24.2-hour cycle. The lowest point of your core body temperature (the “CBT minimum”) falls about 2 hours before your habitual wake-up time. That minimum is the hinge that determines whether light advances or delays your clock:

• Light AFTER CBT-min: advances the clock (makes you a morning person). Use this when flying east.
• Light BEFORE CBT-min: delays the clock (makes you an evening person). Use this when flying west.

Why east is harder than west

Because the human circadian rhythm runs slightly longer than 24 hours, it's easier to extend the day (delay) than to shorten it (advance). That asymmetry is why eastward flights — which require phase-advancing your clock — are the harder direction. The Sleep Foundation and Harvard Health both estimate roughly one recovery day per time zone for eastward trips, but only about two-thirds of a day per zone for westward trips. This calculator uses those rules directly.

The pre-travel head start

You don't have to wait until you land. Shifting your sleep by 30–60 minutes per day for 3 days before departure starts the adjustment early. For an eastward trip, move bedtime and wake earlier each day; for a westward trip, push them later. The schedule above incorporates this automatically and caps the daily shift to a biologically realistic 1 hour east / 1.5 hours west — the body can rarely shift faster than this even with optimal cues.

Melatonin: small doses, well-timed

Endogenous melatonin rises about 2 hours before natural sleep onset. A 0.3–0.5 mg exogenous dose taken 30 minutes before your target bedtime acts as a circadian signal, not a sleeping pill — it tells the brain “it's night.” Larger over-the-counter doses (3–10 mg) don't speed up adjustment and can cause next-day grogginess. Most useful when flying east across 5+ time zones. Always check with a clinician before starting.

Caffeine: a daytime ally, a nighttime saboteur

Caffeine's alerting effect is real and helpful during destination daytime — but its half-life is roughly 5 hours, so a 200 mg dose at noon still leaves ~50 mg in your bloodstream at 10 PM. The conservative cutoff is at least 8 hours before bedtime; some travelers metabolize slowly and need 10–12 hours. The timeline above marks each day's cutoff explicitly.

Sleep hygiene at the destination

• Eat meals on the local clock from arrival — food is a secondary clock cue.
• Get sunlight outdoors in your seek-light window; indoor light is 100× weaker.
• Avoid alcohol on the flight and the first night — it fragments sleep.
• Keep the bedroom cool (60–67°F) and dark; consider an eye mask in summer.
• Short naps (≤ 20 min) early in the destination afternoon are fine if needed.

Medical disclaimer: This calculator is for general wellness and planning purposes only. It is not a substitute for professional medical advice. Consult a healthcare provider before starting melatonin or modifying sleep, light, or caffeine routines — particularly if you are pregnant, have a sleep disorder, cardiovascular disease, or are taking medications. Sources include the Sleep Foundation, Harvard Health Publishing, and the American Academy of Sleep Medicine.