The Science of Jet Lag: Why Flying East Feels Worse Than Flying West

Ask someone who has flown New York to Tokyo and back which leg hurt. They usually name the eastbound one. That matches the clock, not the person’s toughness.

The clock runs a little long

The master clock is a cluster of cells in the suprachiasmatic nucleus. It times sleepiness, the daily drop in core temperature, and the evening rise in melatonin.

Left in constant conditions, with no daylight to reset it, that clock does not run at 24 hours. Czeisler and colleagues measured the free-running period in healthy adults at about 24.2 hours (Science, 1999). Morning light is what pulls it back onto the solar day. Jet lag is the gap you get when a flight moves sunrise faster than that daily correction can follow. You are still sleepy, hungry, and alert on the schedule you left.

Why east is harder

A clock that wants to run long is easier to push later than to pull earlier. Flying west asks you to stay up late. That delay agrees with the drift. Flying east asks you to fall asleep and wake earlier than the clock wants. That advance fights it.

Travel-medicine teaching still uses a rough rule of thumb: about a day of adjustment per zone going east, and a bit less going west. I am not going to pretend that rate was measured to a decimal. It fails once the shift is near 12 hours, because 13 hours east and 11 hours west are almost the same displacement, and it fails for people whose period is not the average. Direction still tells you which morning will be worse. On New York and Tokyo the shift is 13 or 14 hours. Both directions cost most people several days, which is what a half-day shift does once “east is harder” stops being a useful sentence.

Light is the lever with a mechanism

Daylight is the signal the clock uses.

Going east, get bright light in the destination morning and keep the evening dim. Morning light advances the clock. That is the direction you need.

Going west, get bright light in the destination evening and skip it early in the morning. Evening light delays the clock.

The hour matters more than the total amount. A bright hotel lobby at the wrong end of the day moves you the wrong way. A window at the right local hour is the version of this that does not require buying a lamp. I do not sell one, and this page is not a reason to.

Melatonin marks a bedtime

The body already releases melatonin in the evening. A supplement taken near the bedtime you are trying to hit, in the destination, can mark that hour for the clock. The American Academy of Sleep Medicine’s 2007 practice parameters list timed melatonin as a treatment for jet lag, with the clearest case being the eastbound trip where the clock has to advance (Morgenthaler et al., Sleep, 2007). That is a timing signal. It is not a sleeping pill you take at the gate because the flight is boarding.

Taken at the wrong hour, melatonin can move the clock the wrong way. I am not going to print a milligram. The trials do not agree on one dose, and a round number on a web page would pretend they do.

Coffee and a nap are for the meeting, not the clock

A short nap, or a coffee, will get you through a call on the arrival afternoon. Neither one shifts the circadian clock the way timed light does. Use them so you can function. If you use them instead of a light plan, the afternoon feels manageable and you still wake at 3 AM for the rest of the week.

Meals are a weaker signal

There is work on meal timing and the clocks in the liver and gut, which are not the same clock as the one in the brain. Fasting on the plane and eating the first real meal at a destination mealtime is a plausible way to tug those. The human results are mixed, and they are weaker than the light findings. Eat on local time once you land. Do not expect lunch to do morning light’s job.

A two-day trip should often stay on home time

The clock takes several days to move. A two-day trip ends before the shift finishes, and then you pay to shift back. For that length of stay, sleeping and eating on home time is the coherent plan. Adjust when you will actually live on the new clock.

The city-pair notes on this site say the same thing with the hours filled in. The New York and Singapore note, for a trip under four days, is to stay on one clock on purpose instead of splitting the difference and sleeping badly on both. That is travel practice on that route, not a clinical cutoff.

I do not have a trial I will stand behind for a supplement other than melatonin, for a pre-flight plan that moves bedtime by exactly 30 minutes a day for a week, or for a hydration formula. Those claims are common in airport bookshops. They are not in the same category as the Czeisler measurement or the AASM practice parameter.

Before you build a protocol, know the number of hours and the direction. The homepage will show the destination’s local time and date on the day you land. That is the input. The rest is light at the right hour, and enough honesty to leave the rest of the advice in the bookshop.