Jet Lag Calculator

Jet lag severity comes down to how many time zones you cross and which way you go. Your circadian clock drifts back into alignment at roughly one hour per day, and it does that more easily in one direction than the other.

The rule this page uses:

Recovery Days (eastward) ≈ Time Zones × 1.0 Recovery Days (westward) ≈ Time Zones × 0.67

Why east is harder. Flying east shortens your day and demands a phase advance: you must fall asleep earlier than your body wants to. Flying west lengthens it and asks for a phase delay, which means staying up later. Left alone in the dark, the human circadian period runs slightly longer than 24 hours, so delaying is working with the grain and advancing is working against it. Almost everyone finds it easier to stay up two hours than to fall asleep two hours early, and that asymmetry is the whole reason for the two different numbers above.

New York to London, 5 zones east: 5 × 1.0 = 5 days. London to New York, 5 zones west: 5 × 0.67 = about 3 to 4 days.

Same route, same distance, roughly a day and a half of difference in how long you feel wrong.

Crossing the date line. The calculator always takes the shorter way round. Auckland (UTC+12) to Los Angeles (UTC-8) looks like 20 zones on paper, but your body only has to move 4 hours, and it moves them as a phase advance, so it behaves like an eastward trip even though you flew east across the Pacific and gained a day on the calendar. The date changing is a bookkeeping matter. Your clock only cares about the 4 hours.

What actually helps:

• Light at the right time is the strongest lever by a distance. Morning light after flying east, late-afternoon light after flying west. Get it wrong and you push your clock the wrong way.
• Melatonin, 0.5 to 5 mg at local bedtime. The small doses work about as well as the large ones and leave you less groggy.
• Shift your schedule before you go, an hour a day for two or three days. It is the difference between arriving 5 zones out and 2 zones out.
• Hydration is worth doing but is not a jet lag treatment. Cabin humidity sits near 12% against a comfortable 40 to 60%, so you will be dry regardless.

Worst case: any trip of 10 or more zones can leave symptoms running for a week and a half, and going east is where people underestimate it.