Light, food and the honest way to reset

The circadian system is a clock in the hypothalamus that runs slightly longer than twenty four hours and is reset daily, principally by light hitting the retina. Every effective intervention for jet lag works through that mechanism, and every ineffective one is an attempt to override it with discipline.

Why east is harder

The internal clock runs naturally a little over twenty four hours, which means delaying it is easy and advancing it is not. Travelling west lengthens the day and works with the drift. Travelling east shortens it and works against it. The practical consequence is consistent: westward adjustment runs at about ninety minutes a day, eastward at about an hour, which is why a flight to New York is a mild inconvenience and the flight home is three days.

The light rule, which is the whole subject

Light in the morning advances the clock, which is what you want going east. Light in the evening delays it, which is what you want going west. Light at the wrong end of the day actively pushes the clock the wrong way, which is why arriving in Asia and sitting in bright afternoon sun can make the second day worse than the first.

• Going east: seek bright light in the destination morning, avoid it in the destination evening. Sunglasses on the way out of the airport if you land at night and it is bright.
• Going west: seek light in the late afternoon and evening, avoid early morning light for the first day or two.
• For shifts of more than eight hours the rule inverts because you cross the point of maximum sensitivity, which is where a shift calculator earns its keep.
• Outdoor light at ten thousand lux does in twenty minutes what indoor light at three hundred cannot do in three hours. Go outside.

Twenty minutes outdoors at the right hour beats every supplement, every eye mask and every plan to simply stay awake.

Food, which is the second clock

There is a secondary clock in the liver and gut that is set by feeding rather than by light, and it can be shifted faster. A period of twelve to sixteen hours without food ending with breakfast at the destination time gives that system a clear signal, and there is reasonable evidence it speeds the whole adjustment.

The practical version is unglamorous: skip the meal service on an overnight eastbound flight, drink water, and eat breakfast when the destination does. It is also far easier than it sounds, because airline food at three in the morning is a low bar to refuse.

Sleep on the plane, conditionally

Sleep on the aircraft only if it is night at the destination. Sleeping because you are tired, at a moment that is daytime where you are going, reinforces the clock you are trying to move. This is the single most common self inflicted error in long haul travel, and it is committed by people who believe they are being sensible.

If it is daytime at the destination and you are exhausted, a nap of twenty to thirty minutes is fine and anything over ninety is not, because it takes you into deep sleep and you will wake worse than you started.

What actually helps, ranked

Light timing first, by a distance. Meal timing second. Melatonin at a low dose, half a milligram to one milligram, taken in the destination evening, third, and the low dose is genuinely better supported than the higher ones sold everywhere. Caffeine as a tool rather than a habit, in the destination morning only. Exercise, which shifts the clock modestly and reliably improves the first night.

What does not help: alcohol, which fragments sleep badly at altitude and afterwards; sleeping tablets, which produce unconsciousness without shifting anything; and any plan that depends on being tougher than your hypothalamus.

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