Light is the strongest signal for resetting your body clock, and timing is everything. The light exposure planner reads your route and body clock, then lays out a 24-hour timeline for each day: when to seek bright light, when to reduce it, and when to avoid it – so your clock shifts toward destination time instead of away from it.
How does the light exposure planner work?
It is built on the phase response curve – the well-mapped rule for how light shifts the clock depending on when you get it. The pivot is your core body-temperature minimum (CBTmin). Bright light after your CBTmin advances the clock (pulls sleep and wake earlier), which is what an eastward trip needs. Bright light before your CBTmin delays the clock (pushes them later), which is what a westward trip needs.
The planner estimates your CBTmin from your sleep window (the same reading as the body clock calculator), works out which direction you need to shift, and produces per-day windows in destination time: seek light, reduce/dim light in the evening, avoid light when it would push your clock the wrong way, and your target sleep window.
Which factors does it use?
| Direction of shift | Eastward needs morning-side light and evening avoidance; westward is the mirror image. |
|---|---|
| Size of the shift | How many hours the clock must move, which sets how many days of windows you get. |
| Your CBTmin | The pivot the whole plan turns on – seek light on the correct side of it. |
| Adjustment rate | The clock can realistically move about an hour a day advancing (east) or a bit faster delaying (west), so the windows walk forward each day. |
Get the timing backwards – bright light on the wrong side of your CBTmin – and you can push your clock away from local time and make jet lag worse. That is exactly why a planner is more useful than “just get some sun.”
Should I seek light in the morning or the evening?
It depends on direction. Flying east (advancing your clock), you generally want bright light in the destination’s morning and to avoid bright light late in the evening. Flying west (delaying your clock), you want light in the late afternoon and evening and to protect yourself from very early-morning light. In the first day or two after a big shift, the “wrong-side” light zone can fall at an unexpected local hour – the planner marks it so you do not sabotage your own reset. See light exposure for the biology.
Does the type of light matter – sunlight, screens, lamps?
Brightness and timing matter more than the source. Outdoor daylight is far brighter than indoor lighting, even on an overcast day, so a walk outside is the most effective “seek light” action. In the evening, dimming indoor lights and screens supports the “reduce” window before your target bedtime, because bright and blue-rich light suppresses melatonin. Light boxes can substitute when daylight is not available at the right hour, but people with certain eye conditions or bipolar disorder should get medical advice before using bright-light therapy.
How many days of light windows will I get?
Roughly one day of windows per time zone you need to shift, because the clock only moves so fast. A 3-hour shift resolves in a few days; a 8-9 hour shift can take a week or more of consistent timing. The how long does jet lag last guide covers recovery time, and the jet lag calculator gives your overall estimate.
Frequently asked questions
What if I cannot get outside at the right time?
Do what you can: even a bright window or a light box at the “seek” hour helps, and blackout curtains or dark glasses help at the “avoid” hour. Consistency across a few days matters more than any single perfect window.
Can badly timed light make jet lag worse?
Yes. Bright light on the wrong side of your temperature minimum can shift your clock away from local time. That is the reason the planner separates “seek” from “avoid” instead of just telling you to get sunlight.
Do I still need melatonin if I use light?
Not necessarily – well-timed light is powerful on its own. If you are considering melatonin, treat timing as the key variable and speak to a pharmacist or doctor first, especially for children, during pregnancy, or with medication.
Sources
- Khalsa SBS, Jewett ME, Cajochen C, Czeisler CA (2003), Journal of Physiology – a phase response curve to single bright light pulses in humans.
- Eastman CI & Burgess HJ (2009), Sleep Medicine Clinics – how to travel the world without jet lag: applied light and melatonin scheduling.
- U.S. Centers for Disease Control and Prevention – CDC Yellow Book, “Jet Lag” chapter – timed light exposure for adjustment.
- Sleep Foundation – “Light and Sleep” / “Jet Lag” educational overviews.
The direction of the light effect (advance after CBTmin, delay before it) is established chronobiology. The exact window times are jetlag.info’s transparent estimate from your body clock – not a clinical prescription.