1. The Physics of Transmeridian Flight: East vs West#
Jet lag happens when your body clock is still set to your departure time zone while the world around you runs on a new one.
The human clock has an intrinsic period slightly longer than 24 hours (averaging about 24.2 hours). Because of this natural tendency to drift later:
- Flying West (Phase Delay): Lengthens your day. Most people adapt relatively quickly to flying west, at roughly 1 to 1.5 time zones per day.
- Flying East (Phase Advance): Shortens your day. The body finds it harder to move earlier, adapting at only about 1 time zone per day or less.
2. The Pre-Flight Preparation Phase#
If you're crossing more than 4 time zones, you can start shifting before you leave:
- Flying east (e.g. New York → Paris, +6 h): For 3 days before, wake 45 minutes earlier each day; get bright light on waking and avoid bright light after 8 PM.
- Flying west (e.g. London → San Francisco, −8 h): For 3 days before, shift bed and wake 1 hour later each day; get late-afternoon and evening light.
3. In-Flight Fasting: Activating the Food-Entrainable Oscillator#
Your liver and gut contain their own clocks, which respond strongly to meal timing (the food-entrainable oscillator, or FEO).
In animal studies, a fast of around 16 hours followed by a meal can shift food-driven clocks (Fuller et al., 2008, Science). Whether this helps people with jet lag is less certain, but it's low-risk for most healthy adults.
The Fasting-Flight Approach:
- 1.Light or No Meals In-Flight: Skip airline meals or keep them small, and drink water. If you have coffee or tea on board, time it by your destination bedtime, not the cabin's. Skip the fast if you have diabetes, are pregnant or have a history of disordered eating.
- 2.Break Your Fast on Local Time: Eat a balanced, protein-rich meal at normal breakfast time in your destination city. This gives your liver clock a strong cue for the new time zone.
4. Landing Day Light: Seek and Avoid Windows#
The most common mistake travelers make is getting bright light at the wrong time relative to their temperature minimum (Tmin), the low point of the body clock that falls roughly 2 hours before your usual wake time (the body temperature guide explains it).
If your home wake time was 7:00 AM, your Tmin is roughly 5:00 AM home time.
- Light before Tmin delays your clock; light after Tmin advances it. After flying east, early-morning light at your destination can land before your home Tmin and push your clock the wrong way. Wear sunglasses in that early window, then get outside once you're past it.
- After flying west: Get outside in the late afternoon and early evening, and try not to go to bed too early; that light helps push your clock later.
- Caffeine on landing day: Use it to get through the afternoon if you need to, but keep it early enough to clear by your new bedtime.
Work out your home Tmin first; it's the step most travelers skip (more on the avoid-light window).
This is the part ARC automates: when you land, it notices the time-zone change on its own and gives you seek-light and avoid-light windows for each day, worked out from your body clock, so you know when to wear sunglasses and when to get outside. To plan before you fly, try the free jet lag planner.
5. Low-Dose Melatonin (0.5mg to 1mg)#
Many melatonin supplements are 5 to 10 mg. For shifting your clock, much smaller doses appear to work about as well, with less next-day grogginess.
If you use it:
- Traveling East: A low dose (0.5 to 1 mg) around your destination bedtime for the first few nights is a common approach.
- Traveling West: Usually not needed.
Check with a doctor or pharmacist before using melatonin, especially if you're pregnant, take blood thinners, blood-pressure or diabetes medication, or are thinking of giving it to a child. In some countries, including the UK, it's prescription-only.
Educational content, not medical advice. ARC is a wellness app, not a medical device. If sleep problems persist, talk to a doctor.