Jet Lag Calculator — Days to Adjust

Jet lag happens when your body clock is still on your origin time zone while you’re already in another one. A practical rule is that the body takes about 1 day to adjust for each time zone crossed eastward, and a bit less westward. Enter the time zones and direction to see an estimate.

How the estimate is calculated

The calculator uses the National Sleep Foundation’s general rule: approximately 1 day of adjustment per time zone when traveling east, and about two-thirds of that for west. So, six time zones east (for example, Brazil to much of Europe) add up to around 6 days, while the same six time zones west come out to around 4 days.

It’s an estimate, not a fixed rule. Adjustment depends on age, how flexible your sleep is, how much you sleep during the trip, and how you expose yourself to light at your destination. Some people adjust faster, others more slowly.

Why east is worse than west

The body’s internal clock works better when the day is stretched than when it’s shortened. Traveling west lengthens your day (you “gain” hours), and delaying your bedtime is relatively easy. Traveling east shortens your day (you “lose” hours) and requires advancing the body clock — something the body resists more. That’s why the return leg of a trip often feels heavier than the outbound leg, or vice versa, depending on the direction.

How to reduce jet lag

• Light at the right time: for eastward trips, seek morning light at your destination; for westward trips, avoid bright light early in the morning and get afternoon light instead.
• Adjust ahead of time: in the days before, shift your bedtime by 30 to 60 minutes per day in the direction of your destination.
• Switch to local time on arrival: eat and sleep according to the new time zone, even if your body wants otherwise.
• Take care during the flight: stay hydrated, avoid excess alcohol and caffeine, and try to sleep at the times that will correspond to nighttime at your destination.

Jet lag and shift work

The mismatch between the internal clock and the external schedule also shows up in people who change work shifts, without going anywhere. The principles of light, regularity, and gradual adjustment apply to both cases. See the night shift sleep calculator and, if daytime sleepiness is strong, the Epworth sleepiness scale calculator.

This content is for informational purposes and does not replace medical evaluation. If difficulty sleeping persists well beyond what’s expected after the trip, see a professional.