Crossing time zones produces a mismatch between the body's internal clock and local time. The mechanism is well studied and the practical implications are more specific than the general advice suggests.
General information rather than medical advice, and anybody with a condition affected by sleep disruption should be asking a doctor.
Direction matters more than distance
The finding that most people do not know and which changes planning.
Travelling west generally produces less difficulty than travelling east across the same number of zones.
The reason is that the human circadian period runs slightly longer than twenty four hours, so delaying the clock — which westward travel requires — is easier than advancing it.
Practically, a westward trip tends to resolve faster, and an eastward trip of the same distance takes longer and feels worse.
Which is worth knowing when scheduling anything important shortly after arrival.
Light is the main lever
The strongest signal to the body clock, and the one within your control.
Exposure to bright light at particular times shifts the clock in particular directions, and the timing determines whether it helps or hurts.
Broadly, light in the local morning advances the clock, which is what eastward travel requires. Light in the local evening delays it, which suits westward travel.
Getting this backwards actively slows adaptation, which is why wandering around at the wrong hour can make things worse rather than better.
The specific timing depends on how many zones were crossed and in which direction, and published guidance and simple calculators exist for working it out.
The practical protocol
What I actually do, which follows from the above.
Set the watch to destination time on boarding, and start thinking in it immediately.
Sleep on the aircraft only if it is night at the destination, regardless of how tired you are.
On arrival, get outside during the appropriate light window for the direction travelled.
Eat at local meal times rather than when hungry, since meal timing is a secondary signal.
And stay up until a reasonable local bedtime on the first night, which is the part everybody knows and which only works if the light exposure was right.
Naps, more precisely
Where the general advice is too blunt.
The blanket instruction not to nap is not quite right. A short nap early in the local afternoon does not appear to substantially delay adaptation and does address the immediate impairment.
What causes problems is a long nap, or one late in the day, which reduces sleep pressure at night and extends the mismatch.
The workable rule is short and early rather than none at all.
Caffeine and alcohol
Both worth handling deliberately.
Caffeine is useful for managing daytime impairment and has a long half-life, which means afternoon consumption affects night sleep. Timing it to the local morning is the sensible position.
Alcohol is commonly used on flights and works against everything above, since it fragments sleep and produces poor quality rest even when it induces it quickly.
Neither observation is novel and both are routinely ignored on aircraft, generally because a flight is boring.
Melatonin, briefly
Since it is the most commonly used intervention.
There is reasonable evidence for its use in jet lag, particularly for eastward travel, with timing being the critical variable rather than dose.
Its regulatory status differs substantially between countries — available without prescription in some, prescription-only in others — which affects both availability and quality control.
Anybody considering it should be reading proper guidance on timing and speaking to a pharmacist or doctor, since taking it at the wrong hour can shift the clock in the wrong direction.
The short trip exception
Worth stating because it inverts the advice.
For a trip of two or three days across many zones, adapting is counterproductive, since you will have to adapt back immediately.
Staying on home time as far as the schedule allows, scheduling important activity during your own body's daytime, and treating the trip as an extended day is a recognised approach among people who do this regularly.
It only works if the schedule cooperates, and where it does it avoids two adaptations rather than managing one.
The return leg
Frequently harder and rarely planned for.
Having adapted to a destination, returning requires adapting again, in the opposite direction, and the eastward leg of a round trip is the difficult one regardless of which way you went first.
Which means the days after returning are frequently worse than the days after arriving, and scheduling anything demanding immediately on return is a common mistake.
A buffer day before returning to work is worth more than the same day added to the trip, which is not how most people allocate their leave.
Children and the schedule
Worth a note since the standard advice assumes an adult who can override their own tiredness.
Young children adapt on their own timescale and cannot be reasoned into staying awake, which means the family schedule is set by them rather than by the protocol.
What generally helps is prioritising light exposure and meal timing, which work without cooperation, and accepting a slower adaptation.
Building a slow first day into the itinerary is more realistic than expecting everybody to be functional, and it removes the pressure that makes the first day difficult.