The long way round: why a big eastward trip can mean going to bed later
For an eastward trip of ten time zones or more, Jetlag Away moves your body clock later instead of earlier: it delays you by the rest of the day rather than advancing you by the shift itself. At that size the two routes take about as long, and the delay runs with the clock’s natural drift instead of against it.
If you have just opened a plan for Copenhagen to Auckland and been told to go to bed later in the run-up to an eastbound flight, this page is the explanation. It is not a bug. It is a deliberate strategy, and the plan says when it has used it.
Two ways to the same place
The body clock runs on a circle. Landing seven hours east of home means it has to end up seven hours further on, but there are two ways to get there:
- Advance by seven: earlier bedtimes, earlier waking, light in the morning.
- Delay by seventeen: later bedtimes, later waking, light in the evening, all the way round the day until it arrives at the same position from the other side.
For a seven-hour shift nobody would choose the second. Seventeen hours is a long way to go. But the two directions do not move at the same speed, and as the eastward shift grows the gap between the two distances shrinks.
Why the clock prefers to go later
Left to itself, the human body clock runs slightly longer than twenty-four hours, so daylight has to nudge it earlier every day just to hold it in place. Delaying runs with that drift; advancing runs against it. In a laboratory study of a twelve-hour shift, the same people delayed at 2.4 hours a day and advanced at 1.6 [EB09]. Why flying east is harder than flying west covers the asymmetry itself.
Jetlag Away plans with more conservative rates than the laboratory maximum:
| Intensity | Advancing | Delaying |
|---|---|---|
| Gentle | 1.0 h a day | 1.5 h a day |
| Moderate | 1.25 h a day | 1.75 h a day |
| Aggressive | 1.5 h a day | 2.0 h a day |
Where the two routes cross
Put those rates against the two distances and the crossover falls out of the arithmetic:
- Nine hours east. Advancing nine takes 7.2 days at the moderate rate; delaying fifteen takes about 8.6. Advancing wins at every intensity.
- Ten hours east. Advancing ten takes 8 days at the moderate rate; delaying fourteen also takes 8. At gentle the delay is quicker (about 9.3 days against 10); at aggressive it is a third of a day slower (7 against 6.7).
So ten hours is where the two routes tie, and that is the line the plan draws: an eastward shift of ten hours or more is planned as a delay of 24 minus the shift. Below ten, the plan advances you and leans on the nights before departure to do as much of it as possible.
These are the days the plan schedules light for after landing, before counting anything shifted at home. The point is the comparison, not the exact figure.
The researchers do not agree on the line, and that is worth knowing
The crossover is a judgement, and the published guidance draws it in different places:
- Eastman and Burgess advise advancing for up to seven time zones east and delaying for eight or more, while noting the rule “may not always apply to all individual travelers” [EB09].
- Roach and Sargent give an advancing schedule for nine time zones and only recommend delaying at twelve [RS19].
Jetlag Away’s ten sits between them. It is derived from the rates the plan uses rather than quoted from either paper, and how a plan is built publishes the derivation so it can be argued with.
The real danger: a clock that picks its own direction
The strongest reason for planning the long way round deliberately is what can happen when you do not. On a large advance, the clock sometimes gives up and delays anyway, which researchers call antidromic re-entrainment. It is the slowest possible route, and while it is happening every instruction written for an advance is wrong-signed: the morning light meant to pull you earlier lands on the side of the cycle that pushes you later.
Burgess and colleagues point out that shifting earlier before an eastward flight can lower that risk, because it “may also increase postflight exposure to phase-advancing light relative to phase-delaying light” [BC03]. That is why, below the ten-hour line, the plan puts so much weight on the nights before you fly. Above it, the plan stops fighting the direction the clock is most likely to take anyway.
What a long-way-round plan asks of you
Everything in the plan is written for a delay, from the first night:
- Before you fly, your bedtime moves later on the last few nights, by up to an hour a night on gentle, an hour and a half on moderate and two hours on aggressive, and never past the latest bedtime you set. Each later night comes with a light window in the evening before it.
- After you land, the light windows sit on the delaying side of your body’s temperature low: bright light in the evening, and darkness in the hours after the low point, which on a delay can mean sunglasses in the early morning. The core body temperature minimum explains why that hour decides the direction.
- On the Plan screen, a note says the plan has gone the long way round, because the body delays more readily than it advances.
The same route can change strategy with the seasons
The shift is read from each airport’s clock at the moment you are there, so daylight saving time moves it. London to Sydney shows this neatly, because the two cities change their clocks in opposite directions at different times of year:
- In the northern summer, London is on UTC+1 and Sydney on UTC+10: nine hours east, planned as an advance.
- In the northern winter, London is on UTC+0 and Sydney on UTC+11: eleven hours east, planned the long way round as a thirteen-hour delay.
- For a few weeks each spring and autumn the two overlap at ten hours, exactly on the line.
The same flight number can therefore get two opposite plans depending on the date. That is the model working as intended, not two answers to one question.
Sources
- [BC03] Burgess HJ, Crowley SJ, Gazda CJ, Fogg LF, Eastman CI. Preflight adjustment to eastward travel: 3 days of advancing sleep with and without morning bright light. Journal of Biological Rhythms, 2003.
- [EB09] Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Medicine Clinics, 2009.
- [RS19] Roach GD, Sargent C. Interventions to minimize jet lag after westward and eastward flight. Frontiers in Physiology, 2019.
Jetlag Away is a scheduling tool and not medical advice. If you have a sleep or circadian disorder, or any diagnosed condition affected by sleep, talk to a healthcare professional first — see the health disclaimer in our terms.
Last reviewed 10 October 2026. Published 10 October 2026.