Direction decides how hard jet lag hits. The east-west calculator reads your route and tells you whether the flight advances your body clock (eastward, harder) or delays it (westward, easier), how many hours your body clock has to shift, and why the same number of time zones can feel very different depending on which way you fly.
How does the east-west calculator work?
It computes the effective phase shift – not the raw geography. Using real time-zone data, it works out how many hours your clock must move and then takes the shorter way round: if your clock needs to move earlier, that is an advance (eastward); if it needs to move later, that is a delay (westward). This is why some routes surprise people: flying Tokyo to San Francisco sets the clock back 16 hours (17 in the northern winter), which looks like a huge westward jump, but your clock takes the shorter 8-hour (or 7-hour) advance instead, so your body experiences it as an eastward trip – which, across the Pacific and the date line, it geographically is. The calculator shows the body clock shift as its main value (for example “8 h east”) and the raw clock difference in small print underneath (for example “Clock: −16 h”), so the difference is clear.
Which factors does it use?
| Body clock shift | The shorter way your body clock has to move, in hours (at most 12) – this is what your body actually feels. At exactly 12 hours both ways are equally long; the calculator then assumes the easier delay (west). |
|---|---|
| Direction | Advance (east) or delay (west), derived from the sign of that shift, not from the map. |
| Clock difference | The raw gap between the two local times, shown in small print under the body clock shift – useful for the itinerary and the calendar date, but not the difficulty driver. |
Why is flying east harder than flying west?
Because the human clock naturally runs slightly longer than 24 hours, it is easier to stretch the day (delay, which westward travel asks) than to shrink it (advance, which eastward travel asks). Delaying your clock is like staying up late – most people manage it. Advancing it means going to bed and waking earlier than your body wants, which it resists. That is the core reason the jet lag calculator gives eastward trips a higher score, and it is the mechanism explained in why does jet lag happen.
Can a westward-looking flight still be an eastward shift?
Yes – across the date line. Routes like Tokyo to San Francisco or Sydney to Los Angeles set the clock back by 16 hours or more, which reads like a westward shift – yet you fly east across the Pacific, and your clock takes the shorter path, which is an advance. The result is the classic eastbound pattern on those trips: trouble falling asleep at the local bedtime and trouble getting up in the morning. The calculator resolves this for you and labels the direction by what your body clock does, not by compass heading. To see the raw clock gap instead, use the time difference calculator.
Does direction change what I should do about it?
Very much. Eastward (advance) trips call for morning-side light at the destination and an earlier bedtime; westward (delay) trips call for evening light and a later bedtime. Getting light on the wrong side of your body clock can push you the wrong way, so the direction this tool reports is the first input to the light exposure planner. Your chronotype fine-tunes how you personally handle each direction.
Frequently asked questions
Is eastward always worse than westward?
On average, yes, for the same number of zones – but not for everyone. Early “lark” chronotypes sometimes find eastward trips less punishing, and a huge westward shift can still be hard. The calculator reports the direction and the body clock shift; the difficulty comparison is a general tendency.
What is the difference between the body clock shift and the clock difference?
The clock difference is the raw gap between the two local times – flying Tokyo to San Francisco, the clock goes back 16 hours. The body clock shift is the shorter way your internal clock actually moves – here 8 hours forward, eastward – and it is never more than 12 hours. Your body cares about the body clock shift; the clock difference matters mostly for the calendar date.
I am flying north-south – is there any jet lag?
If you stay in roughly the same time zone, there is no clock shift and no true jet lag – only travel fatigue. The calculator will show a body clock shift of close to zero.
Sources
- U.S. Centers for Disease Control and Prevention – CDC Yellow Book, “Jet Lag” chapter – eastward travel generally harder than westward.
- Eastman CI & Burgess HJ (2009), Sleep Medicine Clinics – phase-advance versus phase-delay scheduling.
- Sleep Foundation – “Jet Lag” educational overview on direction of travel.
- NHS – “Jet lag” (nhs.uk/conditions/jet-lag/), last reviewed 31 May 2023.
The direction and the body clock shift are computed exactly from time-zone data. The “east is harder” comparison is established chronobiology applied as a transparent heuristic, not a clinical measurement.