A flight to Bangkok, New York or Sydney is no longer a journey around the world, at least not for the aircraft. Your body sees it differently: its internal clock shifts only slowly, and for a few days sleep, hunger and concentration lag behind local time. That is jet lag. It cannot be avoided completely, but it can be shortened considerably.
What happens during jet lag
Almost every cell in the body follows a rhythm of roughly 24 hours, set by a pacemaker in the brain, the suprachiasmatic nucleus. It is set mainly by light: bright light in the morning moves the internal clock earlier, light in the evening pushes it later. After a flight across several time zones, internal clock and daylight no longer match. Typical consequences are trouble falling asleep, waking early, daytime tiredness, digestive problems and poor concentration.
From about three hours of time difference, most people notice clear symptoms. As a rule of thumb, the body needs around one day per time zone to adjust fully. For Thailand (5 to 6 hours) that is almost a week, for Japan (7 to 8 hours) a good week.
Why flying east is harder
In many people the internal clock naturally runs slightly slower than 24 hours. It therefore finds it easier to lengthen the day than to shorten it. After a westbound flight, e.g. to New York, the day is 6 hours longer: you stay up a little later in the evening and the rhythm follows fairly quickly. After an eastbound flight, e.g. to Asia, you have to go to sleep earlier than your body wants. That works less well, and many people lie awake for a long time in the first nights.
An example: take off from Frankfurt at 21:00 for Bangkok, fly ten and a half hours, and in summer you land at 12:30 local time the next day. For your body it is 7:30 in the morning, after a night in an aircraft seat. There are still ten hours to go before bedtime in Bangkok.
Before the flight
- Shift your bedtime gradually: before an eastbound flight, go to bed 30 to 60 minutes earlier each evening for three to four days and get up earlier. Before a westbound flight, later accordingly.
- Start well rested: boarding tired prolongs the adjustment.
- Choose your flight time: for flights east, overnight flights work well if you can sleep on board. For flights west, a morning departure helps you stay awake until evening at your destination.
During the flight
- Change your watch straight away: think in destination time from the moment you board. The time zone converter shows what time it is at your destination right now.
- Sleep when it is night at the destination: an eye mask and earplugs help. If it is daytime there, try to stay awake.
- Drink plenty, little alcohol: dry cabin air and alcohol both worsen sleep.
After arrival
- Use light deliberately: after flying east, seek morning light and avoid bright light in the evening. After flying west, go outside in the late afternoon and early evening. Daylight is the strongest timer, even on cloudy days much brighter than any room lighting.
- Live on local time consistently: eat meals at local times, even if you are not hungry. Regular meal times support the adjustment.
- Nap briefly, not long: a nap of at most 20 to 30 minutes in the early afternoon is fine. Longer naps during the day push the problem into the night.
- Caffeine in moderation: coffee in the morning helps; from the afternoon on it disturbs night-time sleep.
- Exercise: a walk outdoors combines light and movement.
Melatonin, the body's own sleep hormone, is studied for treating jet lag and recommended by some travel medicine specialists. Whether and in what dose it makes sense for you is something to discuss beforehand with your doctor or pharmacist, especially if you take other medication.
Short trips: better not to adjust at all
If you only stay two or three days, you are often better off staying on your home rhythm. Schedule appointments for times when you would be awake anyway by German time. On a business trip to New York that means morning appointments, because 9 a.m. in New York is 15:00 in Germany. The meeting planner shows the right slot.
Conclusion
Jet lag happens because your internal clock moves more slowly than you do. Going west is easier, going east harder, and as a rule of thumb the body needs one day per time zone. Daylight at the right time, consistent local time and gradual preparation help most. The sleep cycle calculator explains why sleep runs in cycles.
Sources: Centers for Disease Control and Prevention (CDC), Yellow Book, chapter "Jet Lag"; German Sleep Society (DGSM), patient information; specialist literature on chronobiology. This information does not replace medical advice. As of October 2026.