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Everyday questions, clearly explained

Jet lag calculator

Updated

Enter how many time zones you cross and which way you fly. The calculator estimates the days your body clock needs, using the average rates in the CDC Yellow Book, and tells you which light helps.

Your trip

How the calculator works

The CDC Yellow Book chapter on jet lag disorder puts the average adjustment at about 1.5 hours a day after flying west and about 1 hour a day after flying east. The calculator divides the number of time zones by that rate:

  • Flying west: days = time zones ÷ 1.5
  • Flying east: days = time zones ÷ 1

The result is rounded to the nearest half day. West is easier because the average internal clock runs a little longer than 24 hours, so staying up later comes more naturally than going to bed earlier.

Worked example

Flying from Chicago to London crosses 6 time zones going east. At 1 hour a day, that is about 6 days. The return trip west takes about 6 ÷ 1.5 = 4 days.

How to adjust faster

  • Get the right light. Evening light helps after flying west; morning light helps after flying east.
  • Stay awake until local bedtime on the first day and keep naps short.
  • Time your meals and sleep to the destination clock as soon as you can.

Our guide to jet lag after flying west covers what to do before, during and after the flight. For overnight flights, see how to sleep on a red-eye flight.

Limits

These are averages. CDC lists the number of time zones, light at the destination, genetics, medications and other personal factors as things that change how bad jet lag gets. Talk to a doctor or pharmacist before taking melatonin or any sleep aid.

Frequently asked questions

How long does jet lag last?
On average, about 1 day per time zone after flying east and about 1 day per 1.5 time zones after flying west, based on the adjustment rates in the CDC Yellow Book. A 6-hour eastward trip works out to about 6 days.
Is jet lag worse flying east or west?
East. CDC puts the average adjustment at about 1 hour a day after flying east and 1.5 hours a day after flying west.
How many time zones before you get jet lag?
The CDC chapter uses the sleep-medicine definition of jet lag disorder, which applies after crossing at least two time zones.
Does a jet lag calculator replace a sleep plan?
No. This calculator estimates recovery time. CDC notes that plans timing light, caffeine and melatonin to your exact flight times can help on longer trips.

Sources

  1. Jet Lag Disorder, CDC Yellow Book