Ask any frequent long-haul traveller and they'll tell you: flying west is manageable. Flying east is brutal. London to New York: you're tired, a bit fuzzy, but you can work. New York to London: two days of genuine impairment.
It's not just the time zones. It's which direction your biological clock needs to move, and how much resistance it puts up.
The Clock Runs Slightly Long
Here's the key fact: your circadian clock doesn't run on exactly 24 hours. Left to its own devices, in the absence of external time cues, the human circadian clock runs at approximately 24.2 hours — slightly longer than the solar day.
This has a direct consequence for jet lag. Delaying the clock (phase delay) — which is what westward travel requires — runs in the same direction the clock naturally wants to go. It's like pushing water downhill. The clock delays relatively easily.
Advancing the clock (phase advance) — which is what eastward travel requires — works against the clock's natural drift. It's pushing water uphill. The clock resists. It advances more slowly, requires more stimulus, and is more disrupted by mistakes in protocol.
How Many Time Zones Is Too Many?
The general rule: for westward travel, 9–10 time zones can be crossed without the full phase delay becoming unmanageable, because the clock will delay naturally to meet you partway. For eastward travel, beyond 6–7 time zones the phase advance required starts to exceed what the clock can achieve in a reasonable timeframe.
For very long eastward flights — London to Tokyo (9 hours ahead), Los Angeles to London (8 hours ahead) — the body clock may actually phase delay all the way around the long way rather than advancing through the short way. This is why some people coming back from Asia feel the jet lag resolving in an unexpected direction.
For a detailed breakdown of how long recovery actually takes and the variables that affect it, see our guide on how long jet lag lasts.
The Light Protocol for Eastward Travel
To advance your clock after flying east: get bright light exposure — specifically 480–490nm melanopsin-activating light — during the morning at the destination, as early as possible after local sunrise. This is the most powerful circadian phase-advance stimulus available.
Avoid evening light for the first two to three days. Light in the evening drives phase delay — the exact opposite of what you need. Dim hotel room lights after 9pm local time. Use blue light filters on devices from early evening.
For the complete protocol covering both eastward and westward travel, see our science-backed guide to beating jet lag with light.
The LightHealth hotel system does this automatically: it receives your origin timezone at check-in, calculates the phase advance required, and delivers the morning light protocol through the room luminaires across your stay. No protocol to remember. The room simply knows where you came from.
Learn about the LightHealth hotel jet lag system.
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