Jetlag Away

How Jetlag Away builds a plan

Jetlag Away turns a flight into a dated schedule of when to sleep, when to seek light, when to avoid it, and when to stop drinking coffee. This page describes what it does with what you give it. Nothing here is a secret, and knowing the mechanism makes the plan easier to follow — and easier to overrule when you have a reason to.

What it asks for

Two sets of inputs, and both are small on purpose.

The flight. Departure and arrival airports, and departure and arrival times — each one in the local time of its own airport, which is how a boarding pass states them. It also keeps the time zone of each end rather than only the offset, because zones drift relative to one another across daylight-saving boundaries: the same route in July and December is not always the same shift.

You. Your usual bedtime and wake time, the earliest you are willing to be woken and the latest you are willing to be kept up, and how aggressively you want to adjust. The two limits are guard rails — the plan will not schedule you outside them, and they also decide how far it can move your schedule before you fly.

That is all. It does not want your location, your contacts, or an account linked to anything. What it stores and why is set out in the privacy policy.

The two numbers that decide everything

From the flight, the app computes exactly two things, and almost the whole plan follows from them.

The shift is the difference between the two zones’ offsets, measured at the moment you are actually at each end, wrapped into the ±12 range travellers think in. Copenhagen to Tokyo is +7 or +8 depending on the season; Copenhagen to Johannesburg is 0 or 1, despite being a long flight.

The direction is the sign of that number. Positive is eastbound, negative westbound. As flying east is harder explains, this is not a cosmetic distinction — it inverts most of the advice.

Note what is not in that calculation: the flight duration. An eleven-hour flight due south needs no circadian adjustment at all. Tiredness and jet lag are different problems and the app only claims to address the second.

Which way round

A body clock can reach any new alignment by moving in either direction round the day. Flying west, it delays — later bedtimes, later waking, evening light. Flying east, it advances, which is the harder direction because the clock’s own tendency is to drift later.

For the largest eastward shifts the long way round becomes the better route. A plan for an eastbound shift of 10 hours or more moves your clock later, by the rest of the day, rather than earlier by the shift itself: delaying fourteen hours at the delaying rate is no slower than advancing ten at the advancing rate, and it runs with the clock rather than against it. The plan tells you when it has done this, because “go to bed later” on an eastbound trip looks like a mistake unless you know why.

It starts the night you make it

A plan is a schedule, not a list of interventions. From the night after you make it to the end of adaptation, every night has a sleep window. On most of the nights before you fly the instruction is simply sleep at your usual time — those nights post no reminders — and the plan says which nights are the ones that move.

Before departure, the last few nights move your sleep window toward destination time: earlier for an advance, later for a delay, in steps of a quarter hour or more. The size of each step and the total are set by the intensity you chose and by your own limits, and the total is spread over the nights available — a plan made ten days out takes smaller steps over more nights than the same plan made three days out, and a plan made the night before still moves that one night. Each shifted night is paired with a light window on the right side of it, because a schedule shift only moves the clock when the light moves with it.

Intensity Advances at Delays at Shifted before departure Light window Naps
Gentle 1.0 h/day 1.5 h/day up to 1.5 h 30 min up to 30 min
Moderate 1.25 h/day 1.75 h/day up to 3 h 60 min up to 20 min
Aggressive 1.5 h/day 2.0 h/day up to 4 h 90 min none

The two rates are how fast your clock is assumed to move after landing, given that you follow the light windows; they set how many days the plan runs and step the light windows day by day. Gentle uses the rates the clock manages on its own — about an hour a day advancing and an hour and a half delaying, an asymmetry that has been measured since the 1970s. Aggressive uses the rates that timed bright light has been shown to support. Moderate sits between them.

This is a trade, not a quality setting. Notice what the higher settings mostly buy: more of the shift done at home, where you control your light, and longer light windows. The schedule is never advanced by more than an hour a night at any intensity, because advancing it faster does not advance the clock faster — it only pulls sleep out of line with it. The aggressive plan also removes naps deliberately, because sleeping off-schedule feeds the rhythm you are trying to move.

The flight

Sleep on the plane when it is night where you are going, or during your body’s own night, whichever falls inside the flight — eye mask, earplugs, darkness — and stay awake, in the light, the rest of the time. On an overnight flight east that puts the sleep in the later part of the flight so you wake near landing. On an afternoon flight west into an evening it schedules no sleep at all: the task is to stay up until local bedtime.

One rule sits above that. The plan never lets a day run past twenty-four hours between one sleep window and the next, from your last morning at home to your first bedtime on the ground. On a very long journey, or a long daytime flight into an evening, it inserts a short rest where it keeps the stretch to a day.

After you land

Every destination night is at your own usual hours on the local clock, because that is what being adapted means. The days between them carry the light instructions, and this is where the plan does its real work.

The hinge of the light response is your body’s daily temperature minimum, which cannot be felt but can be estimated from your usual bedtime — about five hours after it. Light in the hours before that low point pushes your clock later; light in the hours after it pulls it earlier. On landing, your low point is still where your home clock left it, which on the destination clock can be the middle of the morning or the middle of the evening. Each day the plan estimates where it is, schedules bright light on the correct side of it, tells you to avoid light on the wrong side — sunglasses outdoors, dim rooms — and moves the estimate by the day’s rate. It also tells you the hour it estimated, so you can see the reasoning.

For a Copenhagen traveller landing in Tokyo, that means avoiding bright light for the first hours of the morning and seeking it late in the morning instead. A brisk walk in the sunshine straight off an early-morning arrival is the classic mistake, and it is the one instruction the plan will never give you.

A caffeine cutoff precedes every sleep window the plan sets — six to eight hours before it, by intensity. Naps are offered on the arrival day only, short, and only when they can be over by mid-afternoon.

It adjusts to what actually happened

Plans assume they are being followed, and that assumption is usually wrong somewhere. If you check in during the trip and report how you slept and how you feel, the app re-reads the plan against what you reported rather than against what it originally assumed — a schedule that is not being kept moves the clock more slowly than one that is, and the remaining days are rebuilt on that basis. The nights before the flight stay as they were: a regenerated plan starts where the original did.

This is the part that most distinguishes a plan from a printout. The first day rarely goes as written.

Where the numbers come from

The rates, the step limits and the light rules are not invented. They come from the published circadian literature, and the app’s own documentation records which figure came from where:

What it will not do

It will not promise a number of days. It will not tell you to take melatonin — that is a question for a pharmacist, not a scheduling app. It will not schedule you outside the limits you set, and it will not pretend a two-day eastbound trip is worth fully adjusting for when it may not be.

It is a scheduling tool built on well-established circadian mechanics, applied to your actual flight. That is the whole claim.

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.