How the jet lag calculator works
Jet lag happens because your internal body clock is still set to where you left, while the sun, meals, and social life at your destination run on a different schedule. Your clock can only move so fast — on average it re-syncs at a rate of roughly one timezone per day. Cross six zones and you might feel off for the better part of a week if you do nothing.
This calculator works out the size of the shift, whether you need to advance your clock (go to bed earlier — typical when flying east) or delay it (go to bed later — typical when flying west), and then spreads the change across the days you have before departure at about one hour per day. Pre-shifting at home means you arrive partway adjusted instead of starting from zero.
Why eastward travel is harder
Most people's natural body clock runs slightly longer than 24 hours, so delaying it (staying up later, flying west) feels easier than advancing it (forcing yourself to sleep earlier, flying east). That's why the calculator is a touch more conservative on eastward trips and leans on morning light to help drag your clock earlier.
Light is the real control knob
Of everything that influences your body clock, light is the most powerful. The rule of thumb:
- Flying east (advancing): seek bright light in the morning at your destination and avoid it in the late evening. Morning light pulls your clock earlier.
- Flying west (delaying): seek bright light in the evening and avoid early-morning light. Evening light pushes your clock later.
One important catch: right after a big eastward shift, your body's "biological morning" may still fall in the late local night. Getting bright light at the wrong time can push your clock the wrong way. That's why the plan keeps the shifts gradual.
A note on safety
This is general guidance, not medical advice. If you have a medical condition, are pregnant, take medication affected by timing (such as insulin), or are planning melatonin for a child, talk to a clinician before changing your sleep schedule. This tool does not replace professional advice.
Frequently asked questions
- How long does jet lag last?
- A common rule of thumb is about one day of adjustment per timezone crossed. So a 6-hour shift can take roughly 6 days to fully clear if you do nothing — pre-shifting and good light timing speed that up.
- Is it worse to fly east or west?
- For most people, east is harder. Your natural clock tends to run slightly longer than 24 hours, so delaying it (west) is easier than advancing it (east).
- Should I shift my schedule before I leave?
- Yes, if you can. Moving your bedtime about an hour a day toward your destination's time for a few days before departure means you land already partly adjusted. This calculator builds that plan for you.
- When should I get sunlight to fix jet lag?
- Flying east, seek morning light and avoid late-evening light. Flying west, seek evening light and avoid early-morning light. The calculator tells you which applies to your trip.
- Is SleepShift free?
- Completely. No account, no app to install — it runs entirely in your browser.