What is jet lag and how do I reduce it?
Jet lag occurs when your internal body clock is out of step with local time. That clock governs sleep, alertness, digestion and temperature, and it adjusts by roughly an hour a day, which is why crossing several time zones produces days of disruption rather than one bad night.
Direction matters and is often underestimated. Travelling east is generally harder, because it requires advancing the clock, and the body adapts more readily to delaying than advancing. Westward travel, where the day is lengthened, tends to settle faster.
Light is the strongest tool available, because it is the primary signal the clock uses. Morning light advances the clock, evening light delays it. Getting outdoors at the right time of day for the direction travelled speeds adjustment more than anything else, and getting it at the wrong time can actively slow it.
Adopting local time immediately helps. Eat, sleep and be active according to the destination clock from arrival, rather than calculating what time it is at home. Long naps on arrival feel irresistible and tend to entrench the problem; a short one, kept under about half an hour and early in the day, is less disruptive.
Shifting sleep by an hour or so in the days before departure reduces the gap to close on arrival.
Hydration and moderating alcohol help with how you feel rather than with the underlying clock, which is worth being clear about: they reduce the discomfort of flying without accelerating adjustment.
For short trips of a day or two, many people do better staying on home time rather than adjusting twice.