Jet lag, and why flying east is the harder direction

What follows is the working version of jet lag: the decisions in the order you actually meet them, with the reasoning attached.

A mismatch, not exhaustion

The fatigue of a long flight comes from cramped hours, dry air and disrupted meals, and it clears after one decent night in a bed. Jet lag is a separate problem: your internal clock is still running on the timetable of the place you left, while everything around you runs on local time.

That is why you can feel wide awake at two in the morning after sleeping perfectly well on the aircraft, which is nothing like ordinary tiredness. Digestion, body temperature, alertness and hormone release are all timed by that clock, so hunger and bowel habits go strange alongside the sleep. Understanding it as a timing offset rather than a debt makes the remedies obvious, because you fix an offset by moving it rather than by resting.

Why east is worse

The human clock runs slightly longer than twenty-four hours on average, so it drifts later by default and has to be pulled back each morning by light. Flying west asks the clock to do what it already wants: stay up later and wake later, which is why a westward trip usually settles within a couple of days. Flying east asks for the opposite, an earlier bedtime and an earlier wake, and the clock resists shortening a day far more stubbornly than lengthening one.

A rough working figure used by travellers is around a day of adjustment per time zone crossed going east, and rather faster going west. That asymmetry also explains why the return leg of a trip often feels completely different from the outbound one across the same distance.

Light is the lever, and its timing flips

Light does not simply wake you up; it moves your clock, and whether it moves it earlier or later depends on where your body thinks it is in its own night. Light in your body's subjective morning pulls the clock earlier, while light in your body's subjective evening pushes it later. Because your body clock is still on home time, local morning sunlight after an eastward flight can arrive during your subjective night and shove the clock the wrong way.

Put simply, the practical version is that after flying east it usually helps to seek light later in the local morning for the first day or two, and after flying west to seek it in the local afternoon and early evening. Getting this backwards is the single most common reason a traveller stays misaligned for a week rather than two days.

Short trips are a different calculation

If you are away for two or three days, adapting to local time and then adapting back costs more disruption than simply staying on home time. That means scheduling anything important within your home-time working hours where the local calendar allows it, and accepting odd meal times. For longer stays the calculation reverses, because a week or more misaligned is far more costly than a difficult first two days.

In practice, business travellers who cross zones repeatedly often find that a fixed personal routine, held regardless of location, is easier than adapting each time.

Repeated circadian disruption over years is an area where the evidence is still developing, and frequent long-haul crews are the group most studied.

What to do on the day itself

Setting your watch to the destination as you board is a small act with real effect, because it starts you making decisions in local time immediately. Sleeping on the aircraft is worth it when it falls inside the destination's night and worth resisting when it does not.

In practice, arriving in the local morning and staying upright until a plausible local bedtime is uncomfortable for one day and shortens the whole adjustment. A short nap of twenty minutes or so takes the edge off without eating into the sleep pressure you need for that night. Alcohol on the flight fragments the sleep it appears to bring on, and dehydration makes everything about the first day feel worse.

Some of this will suit you and some will not, and that is the point.

Where this article stops

Medication and supplements sometimes used for travel are outside what this page covers, and their availability and legal status vary considerably between countries. A pharmacist or doctor can advise on those, and it is a sensible conversation to have before a trip rather than in an unfamiliar airport.

People with existing sleep disorders, or on medication timed to a daily schedule, have a genuinely different problem and should get specific guidance before crossing zones. The general strategy of light timing, meal timing and holding to local hours is safe, free and does most of the work for most travellers. The remaining part is patience, since a clock that has to move several hours will take several days no matter how well the trip is planned.

The takeaway

Fly west and your body gets a longer day. Fly east and it gets a shorter one, which it will argue about.

The version you keep doing is the version that works.