Jet Lag Calculator

Jet lag (formally called desynchronosis) occurs when your internal circadian clock is misaligned with the local time at your destination. Your circadian rhythm is an approximately 24-hour biological oscillator regulated by the suprachiasmatic nucleus (SCN) in the hypothalamus. It controls sleep timing, cortisol peaks, digestion, body temperature, and dozens of other physiological processes.

When you cross multiple time zones rapidly by air, your destination's light-dark cycle suddenly conflicts with your body's entrained schedule. The result: you feel alert at 2 AM, exhausted at noon, your digestive system fires at wrong times, and your cognitive performance is measurably impaired. The more time zones crossed, the more severe the misalignment.

Your circadian clock naturally runs slightly longer than 24 hours — closer to 24.2 hours on average. This means it is biologically easier to delay your sleep (stay up later, as in westward travel) than to advance it (go to bed earlier, as in eastward travel).

Eastward travel requires you to fall asleep earlier than your body wants to, which fights your natural rhythm. Research consistently shows eastward jet lag takes approximately 50% longer to resolve than equivalent westward travel. This is why our calculator assigns 1.5 days of recovery per hour when traveling east, versus 1 day per hour heading west.

Light exposure is the most powerful zeitgeber (time-giver) for the circadian system. Light hitting the retina sends signals via the retinohypothalamic tract directly to the SCN, which adjusts melatonin secretion accordingly. The timing of light exposure determines whether your clock advances or delays:

• Light in the morning (after your temperature minimum) advances your clock — helpful for eastward travel.
• Light in the evening delays your clock — helpful for westward travel.
• Avoiding light at the wrong time is equally important — use blackout curtains or blue-light blocking glasses when light would push your clock in the wrong direction.

A practical rule: after crossing more than 6 time zones eastward, avoid outdoor sunlight during the first morning at the destination (your body may interpret it as "evening" light and delay your clock further). Instead, seek bright light in the late morning.

Low-dose melatonin (0.5–1 mg) taken at the destination's target bedtime is one of the most evidence-supported interventions for jet lag. A 2002 Cochrane review of 10 randomised trials found melatonin significantly reduced jet lag scores when taken correctly — but the timing must match the direction of travel:

• Eastward travel: Take melatonin at destination bedtime, starting the night you arrive or even 3 nights before departure if preparing in advance.
• Westward travel: Less evidence supports melatonin use; the natural clock drift works with you.

Consult a healthcare provider before using melatonin, especially if you take other medications or have health conditions.

For trips of 5+ time zones, starting adjustment before departure can meaningfully reduce recovery time:

• Eastward: Shift bedtime 1 hour earlier for 3 nights before departure. Seek morning light each day.
• Westward: Shift bedtime 1 hour later for 3 nights before departure. Seek evening light.
• Hydration: Cabin air humidity is typically 10–20%, well below optimal 40–60%. Dehydration worsens fatigue and disrupts sleep. Drink 250ml of water per hour of flight.
• Alcohol: Avoid inflight alcohol — it increases arousals and reduces sleep quality significantly, compounding jet lag.
• Caffeine timing: Use caffeine strategically. During an overnight eastward flight, avoid it in the last 8 hours of flight so you can sleep at destination bedtime.