When you cross multiple time zones, your wall clock jumps instantly, but your biological master clock in the suprachiasmatic nucleus (SCN) shifts by only about one hour per day.
Almost every travel guide tells you to "get sunlight when you land." But chronobiologists know that timing is everything. If you step into bright light at the wrong biological hour, you send a phase-delay signal to a clock that needs a phase-advance—making your jet lag significantly worse.
The Circadian Phase Response Curve (PRC)#
Your body clock's response to light isn't fixed; it follows a Phase Response Curve (measured in humans by Khalsa et al. 2003, Journal of Physiology):
- 1.Phase Advance Window (Morning biological time): Light exposure in the hours following your core body temperature minimum (about 2 hours before natural wake time) pushes your clock earlier (eastward travel).
- 2.Phase Delay Window (Evening biological time): Light exposure in the hours before your core body temperature minimum pushes your clock later (westward travel).
- 3.The Dead Zone: Light exposure during biological midday has very little shifting power on master clock timing.
Why the "Avoid-Light" Window Matters#
If you fly from New York to London (a 5-hour eastward jump) and land at 7:00 AM London time, it is only 2:00 AM biological body time.
If you step outside into bright morning light in London while your body clock is still at 2:00 AM, the photons hit your retina during your Phase Delay zone. Instead of advancing your clock to London time, the light tricks your brain into thinking it is yesterday evening, pushing your clock the wrong way, further from London time.
Finding Your Temperature Minimum (Tmin)#
The hinge of the whole curve is your core body temperature minimum, or Tmin: the point in the 24-hour cycle when your core temperature is at its lowest. For most people it falls about two hours before their natural wake time. If you naturally wake at 7 AM, your Tmin is around 5 AM.
Light after your Tmin pulls your rhythm earlier (a phase advance); light in the hours before it pushes your rhythm later (a phase delay). The same rule works at home, not just after a flight: it's how you shift toward an earlier schedule, or adjust to a new work pattern, without feeling miserable. The core body temperature and Tmin guide covers the temperature side in more depth.
The Protocol: Seek vs. Avoid#
To shift your rhythm as fast as you realistically can:
- Identify Your Tmin: Usually about 2 hours before your habitual wake time (on your home clock, until you've started to shift).
- Wear Sunglasses and Stay Indoors During the Avoid Window: Block bright sunlight and keep indoor lighting dim during your personal delay zone. If you're driving, ordinary sunglasses are fine; don't drive in very dark lenses, and don't drive drowsy.
- Get Outside During the Seek Window: Once past your Tmin, step outside for about 20 minutes of daylight (longer under heavy cloud) to lock in the phase advance. Never look directly at the sun.
Want to see the windows for a specific trip? Plan it with our free Jet Lag Planner, or read the full travel jet lag guide.
Automatic Jetlag Detection in ARC#
You don't have to work out your Tmin by hand. ARC notices international travel without requiring location or GPS permissions, by reading your iPhone's time zone change when you land. It separates wall clock from body clock and gives you both a Seek-Light and an Avoid-Light window in the app, and it handles daylight-saving shifts the same way.