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Jet lag calculator: how many days to adjust?
Short answer
On average the body clock adapts about 1.5 hours a day after flying west and 1 hour a day after flying east, according to the CDC. Crossing 6 time zones that means roughly 6 days to adjust flying east and 4 days flying west. Westward trips are usually easier because the body clock delays more easily than it advances.
Last reviewed 28 October 20261 published sourcesNot medical advice
Reps keeps track of this for you on iPhone, iPad and Apple Watch.
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Count the time zones between home and your destination, and whether you fly east or west.
Enter how many time zones you cross.
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| Time zones crossed | Flying east | Flying west |
|---|---|---|
| 2 | 2 | 1.3 |
| 3 | 3 | 2 |
| 4 | 4 | 2.7 |
| 5 | 5 | 3.3 |
| 6 | 6 | 4 |
| 7 | 7 | 4.7 |
| 8 | 8 | 5.3 |
| 9 | 9 | 6 |
| 10 | 10 | 6.7 |
| 12 | 12 | 8 |
Where the numbers come from
The CDC Yellow Book says westward travel is easier to adapt to than eastward travel, "with an average rate of adaptation of 1.5 hrs/day for westward travel and 1 hr/day for eastward travel" (CDC Yellow Book 2026). The calculator divides the time zones you cross by that rate.
Jet lag disorder is defined around sleep problems after travel across at least two time zones, and for most people it is easier to delay the body clock than to advance it (CDC Yellow Book 2026). These are averages: individual recovery varies, and light exposure and sleep timing around the trip change the pace.
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Common questions
How long does jet lag last?
The CDC gives an average adaptation rate of 1.5 hours a day for westward travel and 1 hour a day for eastward travel. A trip across 6 time zones therefore takes about 4 days to adjust to going west and 6 days going east, with wide variation between people.
Is jet lag worse flying east or west?
East, for most people. The CDC notes that westward travel is easier to adapt to, because for most people it is easier to delay the body clock than to advance it.
How many time zones cause jet lag?
The diagnostic criteria for jet lag disorder involve sleep problems linked to travel across at least two time zones.
Sources
Figures on this page were checked against these publications on 28 October 2026.
- Riedy SM, Williams SG. Jet lag disorder. In: CDC Yellow Book 2026: Health Information for International Travel. Centers for Disease Control and Prevention; 2025. www.cdc.gov/yellow-book/hcp/travel-air-sea/jet-lag-disorder.html
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