“One day per time zone” is the rule of thumb everyone quotes. Fly from London to New York (5 hours behind) and expect 5 days to recover. Fly to Tokyo (9 hours ahead) and expect 9 days. It's a reasonable heuristic, but it significantly underestimates the range of individual variation — and it ignores what you can do about it.
What Determines Recovery Time
Three factors matter more than the number of time zones.
Direction of travel. As we covered in our guide to east vs west jet lag, eastward travel is harder than westward because it requires phase advance, which the circadian clock resists. Westward recovery is typically faster than the rule of thumb suggests; eastward recovery is typically slower.
Your individual circadian clock speed. People with faster-running clocks (morning chronotypes, sometimes called “morning people”) adapt more quickly to eastward travel because their clock already runs slightly faster. People with slower clocks (evening chronotypes) adapt more slowly eastward but faster westward.
How well you use light. This is the variable you can control. The circadian clock advances in response to morning light and delays in response to evening light. The strength and timing of the light stimulus you receive determines how quickly re-entrainment occurs.
What the Research Shows on Light Protocols
Studies from the Brigham and Women's Hospital in Boston and multiple other circadian research groups have shown that structured light therapy can reduce jet lag recovery time by 30–50% compared to unmanaged recovery. For a 9-hour eastward trip where unmanaged recovery might take 7–9 days, structured morning light can reduce this to 4–5 days.
The protocol: bright light exposure (preferably outdoor, or 480–490nm targeted light) in the first 60–90 minutes after local sunrise at the destination, combined with evening light avoidance (dim lights and blue light filters after 9pm local time). Consistent application for the duration of the stay. For the step-by-step guide, see our complete beat-jet-lag guide.
What Doesn't Help as Much as You Think
Melatonin at destination bedtime helps you sleep at the right time. It doesn't advance the clock directly. It's a sleep aid, not a re-entrainment tool. Useful in combination with light, but not a substitute for it.
“Staying up until local bedtime” is good advice, but only works efficiently if combined with correct morning light. Without the morning light stimulus, staying up late just means you're tired and your clock is still misaligned.
The hotel room matters more than people realise. A room with blackout curtains that block morning light, and bright artificial lighting in the evening, actively worsens jet lag. This is the jet lag paradox: the standard hotel room environment is optimised for the wrong biology.
Learn how LightHealth solves this at the hotel level.
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