Jet Lag Calculator

What Is Jet Lag?

Jet lag is the mismatch between your body's internal clock and the local time at your destination. Your circadian rhythm controls when you feel alert, hungry, and sleepy, and it only drifts by roughly one hour per day. When a flight moves you across several time zones at once, your body is still running on home time while the world around you has already moved on. The result is the familiar mix of insomnia, daytime fatigue, poor concentration, and appetite disruption that follows long eastwards or westwards flights.

This calculator turns that vague feeling into numbers. It computes the time zone shift between your origin and destination, rates the jet lag severity, estimates how many days you need to adapt, and builds a night by night sleep schedule that gradually walks your bedtime to the destination clock.

How the Calculation Works

The tool first looks up the real UTC offset of both time zones on your travel date, so daylight saving time is handled automatically. It then reduces the difference to the shortest direction. Crossing 14 hours east will almost always be easier when treated as 10 hours west, and the calculator applies that adjustment for you.

Once the effective shift is known, three estimates follow:

• Severity rates the disruption from minimal (under one hour of effective shift) through mild, moderate, and significant, up to severe (nine hours or more).
• Recovery time applies an adaptation rate of about one hour per day for eastward travel and about 1.5 hours per day for westward travel, reflecting that delaying the body clock is easier than advancing it.
• Adjustment schedule shifts your usual bedtime and wake time by the remaining clock difference each night, until both settle onto destination time.

Why Eastward Travel Is Harder

Human circadian rhythms naturally run slightly longer than 24 hours. Delaying the clock, which is what westbound travel asks for, fits that tendency. Advancing the clock, required by eastbound travel, forces your body to do the opposite of what it prefers. Studies of transatlantic crews and travellers consistently find that eastward adaptation takes longer and produces more severe symptoms than westward travel of the same size, which is why the recovery estimate and the pre flight advice in this tool are direction aware.

Reading the Adjustment Schedule

The schedule table shows what your body clock needs each night, expressed in destination local time:

• Arrival night reflects how your usual bedtime maps onto the destination clock before any adaptation. An 11:00 PM bedtime that lands at 7:00 AM destination time is the clearest signal that eastward travel will be rough.
• Following nights move the recommended bedtime and wake time by the adaptation rate until the body clock status reads "on destination time".
• Body clock status tells you how far ahead of or behind destination time you still are, so early nights in the schedule do not need to feel forced.

For very small shifts the schedule may contain only a single arrival night, because there is nothing meaningful to adjust.

Practical Recovery Tips

• Use light strategically. Eastbound travellers should get bright morning light and dim evenings. Westbound travellers should seek late afternoon and evening light while keeping mornings dim.
• Start before you fly. Moving bedtime 30 to 60 minutes in the direction of travel for two or three days before departure shortens recovery by a day or more.
• Set your clocks early. Switch your watch, phone, and meal schedule to destination time as soon as you board.
• Hydrate, and go easy on alcohol. Cabin air is dry and alcohol fragments sleep, both of which make the first nights worse.
• Nap short, not long. A 20 to 30 minute nap on arrival day can carry you to a normal local bedtime. Long naps steal the sleep pressure you need that night.
• Anchor with meals. Eating at local meal times is one of the strongest signals for resetting the circadian clock.

Limitations

Jet lag severity varies from person to person. Age, chronotype (morning lark or night owl), sleep debt, hydration, and whether you can sleep on the plane all change the outcome. The recovery estimate here describes a typical adult following reasonable light and sleep habits, and it is not medical advice. If you have a sleep disorder or take medication that affects sleep, talk to a clinician before relying on any travel schedule.

Privacy

Everything is calculated in your browser. Your time zones, travel date, and sleep schedule are never uploaded or stored, and the tool works offline once the page has loaded.

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• Sleep Debt Calculator
• Convert Time Between Timezones
• REM Sleep Calculator