Jet lag isn't really caused by the flight itself — it's caused by your circadian rhythm, the internal roughly-24-hour clock that governs sleep, hormone release and alertness, being suddenly out of sync with the light-dark cycle of a new timezone. A long flight that doesn't cross many timezones, like a north-south route, produces very little jet lag no matter how tiring the flight feels, while a shorter flight that crosses six or eight timezones east-to-west can leave you off-kilter for the better part of a week. This calculator's estimate is driven by the size of the timezone shift, not the flight duration.
The human circadian rhythm left to its own devices tends to run slightly longer than 24 hours, which is why it's generally easier for your body to accept a longer day than a shorter one. Travelling westward effectively lengthens your day, working with that natural tendency, while travelling eastward shortens it, working against it. This is the well-documented reason eastward jet lag — flying, say, from the Americas to Europe or Asia — tends to feel worse and take longer to shake off than the equivalent westward trip, even across an identical number of timezones, which is exactly why this calculator applies a longer estimated recovery for eastward travel.
A commonly cited rule of thumb among sleep researchers is that the body adjusts to a new timezone at a rate of roughly one day per hour of time difference when travelling eastward, and somewhat faster — often cited around two-thirds of a day per hour — when travelling westward. This is an average across a large range of individual variation; some people adjust faster than average and some slower, and factors like age, sleep quality on the flight, and how deliberately you manage light exposure after arrival all shift the real number in either direction.
Light exposure timed to your destination's schedule is the single most effective lever available, since light is the primary signal your circadian rhythm uses to reset itself — seeking bright light in the morning at your destination if you're trying to shift earlier, or in the evening if you're trying to shift later, speeds adjustment more than almost anything else. Shifting your sleep schedule by an hour or so per day in the days before departure, when practical, gives your body a head start rather than asking it to make the full jump all at once on arrival. Avoiding a long nap on your arrival day, however tempting, and holding out until local night-time helps anchor your new sleep schedule faster than an uncontrolled multi-hour daytime sleep would.
Once you know roughly how many rough days to expect, it's worth keeping the first day or two of your Travel Itinerary Planner light on demanding plans, since trying to pack in a full sightseeing day while still jet-lagged rarely goes well.
One more thing worth knowing: naps taken strategically — short, timed early in the local afternoon rather than long and unplanned — can help manage fatigue during the adjustment period without derailing the process of resetting your sleep schedule to local time the way an uncontrolled multi-hour nap can.
It's the general pattern researchers observe on average, tied to the body clock's natural tendency to run slightly over 24 hours, but individual experience varies — some people notice little difference between directions.
Not directly. A long flight with little timezone change, like a long north-south route, causes minimal jet lag, while a shorter flight crossing many timezones can cause significant jet lag despite the shorter time in the air.
Deliberate light exposure timing, adjusting your sleep schedule before departure, and avoiding long daytime naps on arrival can meaningfully speed adjustment for many people, though there's a practical limit to how much any single trip's jet lag can be shortened.
No — the calculation is based purely on the timezone offset difference between the two cities, not on which calendar date you land, so the date line itself doesn't change the jet lag estimate.