The Jet Lag Light Exposure Planner, Explained
After reading this you will know why light, not sleep, resets your body clock, how to work out whether to advance or delay it, and how to time bright light and darkness around your body-temperature minimum for each day of a trip.
What the planner does
Your body runs on an internal clock with a period close to 24 hours. When you fly across time zones, that clock stays on home time while the sun, meals and social life switch to destination time. The mismatch is jet lag: wrong-time sleepiness, wrong-time alertness, dull digestion and low mood until the clock catches up.
The strongest lever for moving the clock is light hitting your eyes at the right time. The Jet Lag Light Exposure Planner takes your home and destination UTC offsets and your usual sleep hours, decides whether you need to shift earlier (advance) or later (delay), then prints a day-by-day schedule of when to seek bright light, when to avoid it, and when melatonin might help, all in destination local time.
Here is the hook. Fly from London (UTC 0) to Tokyo (UTC +9) and you must move your clock 9 hours earlier. At roughly 1.5 hours of advance per day, full adjustment takes about 6 days. Fly the same route home and you delay 9 hours at up to 2 hours per day, which needs about 4 to 5 days. Same distance, different difficulty, because your free-running clock is slightly longer than 24 hours and prefers to delay.
When to use it, and when not to
Use the planner for trips that cross at least 3 time zones and that last long enough to be worth adjusting, usually 4 nights or more. For a 2-day hop, staying on home time and sleeping when you can is often less disruptive than a half-finished shift.
The plan assumes healthy adult circadian physiology, ordinary daylight, and freedom to control your light. It does not model shift work, night flights with no sleep, or the effect of alcohol and heavy meals. It ignores daylight saving: the offsets you enter are treated as fixed.
Melatonin, light boxes and sleep timing interact with medication and with conditions such as bipolar disorder, seizure disorders and eye disease. This article is educational, not medical advice. Ask a professional before you change anything that affects your health.
The direction rule and the shift size
First decide the direction. Compute the raw time-zone difference, then fold it into the range from -12 to +12 hours, because a 9-hour eastward shift and a 15-hour westward shift land on the same clock.
Here \text{dest} and \text{home} are the two UTC offsets in hours, and \Delta is the required shift. A positive \Delta means the destination clock is ahead of home, so you travel east and must advance (go to bed and wake earlier). A negative \Delta means you travel west and must delay.
Next, estimate how many days the shift needs. Advancing moves at about 1.5 hours per day, delaying at up to 2 hours per day:
D_{\text{needed}} is the number of days to finish the shift. If you allow fewer days than this, the plan pushes the clock as far as it can, and you arrive partly adjusted. Those per-day rates are population averages with wide spread: real people shift anywhere from about 1 to 2.5 hours per day depending on age, light dose and genetics.
Timing everything around your temperature minimum
Light does not always push the clock the same way. Its effect depends on when it arrives relative to your core body-temperature minimum, written T_{\min}. That minimum sits roughly 2 to 3 hours before your usual wake time. With a 07:00 wake time, estimate T_{\min} near 04:30.
The rule that drives the whole plan is the phase response curve: light just after T_{\min} advances the clock (pulls it earlier), and light just before T_{\min} delays it (pushes it later). So the same morning sun helps an eastward traveller and hurts a westward one.
- To advance (eastward): seek bright light in the window from T_{\min} to about 3 hours after it. Avoid light for the 3 hours before T_{\min}.
- To delay (westward): seek bright light for the 3 hours before T_{\min}. Avoid light in the 3 hours after it.
Because the clock moves each day, T_{\min} moves too. On an advancing plan it slides about 1.5 hours earlier per day; on a delaying plan it slides up to 2 hours later per day. The planner recomputes the light windows against the new T_{\min} for every day, which is why the schedule creeps across the clock rather than repeating.
Bright means bright. Outdoor daylight is 10,000 to 100,000 lux even under cloud. A well-lit room is often only 200 to 500 lux, ten to a hundred times weaker. Getting outside beats sitting by a window.
A worked example: London to Tokyo in 4 days
Reproducing the demo
The demo uses the defaults: home offset 0, destination offset 9, bedtime 23:00, wake time 07:00, and 4 days to adjust.
- Direction: \Delta = ((9 - 0) + 12) \bmod 24 - 12 = 21 \bmod 24 - 12 = 9 hours. Positive, so advance and travel east.
- Days needed: \lceil 9 / 1.5 \rceil = 6 days for a full shift. You have only 4, so you will finish partly adjusted.
- Progress in 4 days: 4 \times 1.5 = 6 hours of the 9-hour shift, leaving 3 hours of residual lag on arrival.
- Starting temperature minimum at home rhythm: T_{\min} \approx 07{:}00 - 2{:}30 = 04{:}30 home time.
- Day 1 light windows to advance: seek light 04:30 to 07:30, avoid light 01:30 to 04:30. Each following day both windows move about 1.5 hours earlier as T_{\min} shifts.
Expressed in destination time, those windows land in the Tokyo morning, exactly when stepping outside is easy. The residual 3 hours clears on its own over the next couple of days.
Reading the plan and the phase response curve
The widget below lets you slide your temperature minimum and watch the light windows flip sides. This is the one idea a static picture cannot carry: the same clock time is helpful or harmful depending on which side of T_{\min} it falls.
When you read your own plan, focus on three things. The direction tells you which way your whole day must move. The seek-light window is where to earn your daily shift, so protect it. The avoid-light window is where a single bright mistake can undo the day, so wear dark glasses or stay indoors then.
Common mistakes
The failure that wastes the most days is getting light on the wrong side of T_{\min}. If you fly east to advance but catch bright light in the 3 hours before your temperature minimum, you delay instead. That is not a small loss: a strong wrong-time dose can push you an hour the wrong way, turning a +1.5 hour day into roughly a +0.5 hour day or worse.
A second mistake is treating dim indoor light as enough. At 300 lux the phase shift is a fraction of what full daylight delivers, so the plan's 1.5 hours per day quietly becomes 0.5. Get outside.
A third is ignoring the residual on short trips. In the worked example you finish 6 of 9 hours. Expect to still wake early and fade early for a day or two, and do not read that as a failed plan.
A fourth is expecting exactness. The 1.5 and 2.0 hour per day rates are averages. If you shift at 1.0 hour per day, a 9-hour trip needs 9 days, not 6. Watch your own sleepiness and adjust rather than trusting the calendar.
Related tools
Once your target sleep time settles, the Sleep Cycle Calculator helps you pick a bedtime or alarm that lands between 90-minute cycles instead of mid-cycle. Caffeine is a common jet-lag crutch, and the Caffeine Half-Life Visualizer shows how much is still in you at your new bedtime, which matters more when your clock is fragile. If you are travelling to study or present, the Spaced Repetition Simulator shows why spacing your review across the adjustment days beats one jet-lagged cram session.
Frequently asked questions
Should I start shifting before I fly?
You can, and it shortens the lag on arrival. Advancing your sleep and light by an hour a day for two or three days before an eastward trip means you land with less of the 9-hour gap left. The same direction rule applies at home: seek light after your temperature minimum to advance.
When does melatonin fit in?
Low-dose melatonin taken in the evening (relative to your target clock) nudges the clock in the same direction as evening darkness and reinforces an advance. Timing and dose matter and interact with medication, so treat the planner's melatonin window as a prompt to ask a professional, not a prescription.
Why is flying east worse than flying west?
Your free-running clock period averages a little over 24 hours, so delaying (staying up later) is easy and advancing (going to bed earlier) fights your natural drift. That is why the model uses up to 2 hours per day west and only about 1.5 hours per day east.
Do I really need to avoid light, or just seek it?
Both. Wrong-time light actively pushes the clock the wrong way, so a bright hour in the avoid window can cancel a correct hour in the seek window. On the days that matter most, protecting the avoid window with dark glasses is as valuable as chasing daylight.
Does the plan work for a 2-day trip?
Usually not worth it. In 2 days an eastward shift reaches only about 3 of a needed 9 hours, so you spend the whole trip adjusting and then re-adjusting at home. For very short trips, keeping home hours often costs less.
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