The Avoid-Light Window: The Missing Half of Jetlag Science

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 retina's sensitivity to light is not static; it follows a Phase Response Curve:

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 circadian clock back toward California!

The Protocol: Seek vs. Avoid

To shift your rhythm as fast as humanly possible:

Automatic Jetlag Detection in ARC

ARC detects international travel automatically without requiring location or GPS permissions by reading the device's UTC offset change upon landing. It separates wall clock from body clock and gives you both a Seek-Light and an Avoid-Light countdown right on your lock screen.