Light timing

If you only manage one thing on a trip across time zones, make it light. It is the strongest signal the circadian system takes its cue from, and it is the only one you can control precisely without a prescription. It is also the one most easily got backwards, because the same walk outside can move your clock in either direction depending on nothing but the hour you take it.

The same light, opposite effects

The body clock does not simply respond to how much light you get. It responds to when, and the response reverses partway through the cycle.

The hinge is your lowest point of core body temperature — a trough that falls in the small hours, typically a couple of hours before you would naturally wake. It is not something you can feel directly, but its position is predictable from your usual sleep schedule, which is why Jetlag Away asks for your normal bedtime and wake time rather than guessing.

• Light in the hours before that low point delays the clock. It pushes your body later — later sleep, later waking. This is what you want when flying west.
• Light in the hours after it advances the clock. It pulls your body earlier. This is what you want when flying east.

Get this the wrong way round and you do not merely waste the effort. You actively drive the clock away from local time, and the trip gets worse rather than better. The classic version is landing in Tokyo from Europe early in the morning, feeling terrible, and going for a long walk in bright sunshine to wake up — which, at that hour relative to your unshifted body clock, is a large dose of the delaying signal when you needed the advancing one.

This is the single most useful thing on this page: there is no such thing as light being generally good for jet lag. There is only light at the right hour or the wrong one.

Darkness is half of it

Because the response curve has two sides, avoiding light is as much a lever as seeking it. A plan that tells you when to get light and says nothing about when to avoid it is doing half the job.

Avoiding light is harder than it sounds on a trip, because the hours you most need to protect are often the hours the world is brightest. Practical measures are unglamorous and effective: sunglasses on arrival if you land into the wrong part of your own cycle, an eye mask on the plane, blackout curtains, and keeping the room genuinely dark rather than dim.

What counts as bright

Outdoor daylight is far brighter than indoor lighting, and the gap is much larger than it looks — the eye adapts so well that a well-lit office and an overcast street feel comparable and are not. Even a grey day outside is worth substantially more, as a circadian signal, than sitting by a window indoors.

That gives one simple rule: when the plan says seek light, go outside. Thirty to ninety minutes of it, depending on how fast you are trying to shift. Sitting near a window is a weak substitute, and an ordinary lit room is weaker still.

Duration matters, but less than timing. An hour at the correct hour beats three hours at the wrong one, and beats it by moving your clock in the right direction rather than the wrong one.

Screens are not the main event

Phone and laptop screens are frequently blamed for sleep problems, and they do emit light at wavelengths the circadian system is sensitive to. But the quantities are small compared with daylight, and the practical significance of an evening spent on a phone is modest next to the practical significance of when you saw the sun.

Which is not permission to ignore them. In the hours you are trying to protect — when you are attempting to sleep earlier than your body wants — screens are one more delaying signal at exactly the wrong time, and they are the easiest one to remove. Treat them as the last few percent, after you have got the daylight right, rather than as the problem.

Melatonin, briefly

Melatonin is the other well-known circadian lever, and it has its own timing response that is roughly the mirror of light’s. It is regulated differently from country to country — prescription-only in much of Europe, sold freely elsewhere — and dose and timing both matter.

Jetlag Away does not schedule it, and this page will not tell you how to take it. It is a supplement question for a pharmacist or a doctor, not a scheduling question for an app. The app builds its plans on light, sleep timing and caffeine, which are the three you can act on without anybody’s permission.

How Jetlag Away uses this

Every plan the app builds is, underneath, a sequence of light instructions arranged around the direction you are travelling and where your body clock currently sits. It estimates your temperature low from your usual bedtime — about five hours after it — and, for each day of the plan, works out where that low point falls on the local clock: still where your home clock left it on the first day, and moving by the day’s rate after that. Light is then sought on the correct side of it, for thirty, sixty or ninety minutes depending on the intensity you chose, and avoided on the wrong side. The instruction names the hours and the estimate they came from, so “avoid bright light until 11:30” arrives with the reason.

The same rule runs before you fly. Each night the plan moves your sleep window is paired with a light window on the right side of it — the morning after an earlier night, the evening before a later one — because the schedule shift only moves the clock when the light moves with it. On the flight itself an advancing plan asks you to stay awake and seek light in the first half and sleep in the second; a delaying plan, the reverse.

How a plan is built walks through the rest of the calculation.

Jetlag Away is a scheduling tool and not medical advice. If you have a sleep or circadian disorder, or any diagnosed condition affected by sleep, talk to a healthcare professional first — see the health disclaimer in our terms.