☕ Last Coffee11:30 AMOne ~100 mg cup, average sensitivity. A bigger cup moves it earlier.
Day 3Sleep Window: 6:00 PM - 2:00 AM
Day 3: your body clock is about 5 h ahead of local time. Seek bright late-afternoon and evening light, and keep the very early morning dim.
☀️ Seek Light12:00 PM - 5:00 PMStep outside, no sunglasses. Never look at the sun.
🕶️ Avoid Light / Dark12:00 AM - 4:00 AMDim screen, use blackouts.
☕ Last Coffee12:30 PMOne ~100 mg cup, average sensitivity. A bigger cup moves it earlier.
By day 4 you'll still be about 4 h off local time. Keep shifting about an hour a day.
Educational, not medical advice. If sleep problems persist, talk to a doctor.
Let ARC notice the time zone change
When you land, ARC notices your iPhone's time zone changed (no location permission, nothing to type) and gives you a seek-light and an avoid-light window each day while your body clock catches up, about an hour a day.
Jet lag happens when your body's master clock, the suprachiasmatic nucleus (SCN) in the brain, is out of step with the day at your destination. The wall clock jumps the moment you land; your body clock moves about an hour a day.
Phase Advance vs. Phase Delay
Depending on which direction you fly, your body needs to shift its clock in one of two directions:
Phase Advance (Flying Eastward): You need to shift your bedtime and wake time earlier. Get bright light in your body's morning. For the first days after an eastward flight that's late morning to early afternoon at your destination; keep the early destination morning dim.
Phase Delay (Flying Westward): You need to shift your bedtime and wake time later. Get bright light in the destination late afternoon or evening, which pushes your melatonin rise later and helps you stay awake, and keep the very early morning dim.
What is Tmin (Temperature Minimum)?
Your core body temperature follows a 24-hour cycle and reaches its lowest point (Tmin) roughly 2 hours before your natural wake time. Light after Tmin advances your clock (earlier); light before Tmin delays it (later). That's why the planner puts your seek-light window after your body's Tmin when you fly east, and why its avoid-light window matters as much as the seek-light one.
Frequently asked questions
Is jet lag worse flying east or west?
For most people, east. Flying east means your body clock has to move earlier (a phase advance), and the human clock shifts later more easily than earlier, so westward trips usually settle faster.
How long does jet lag last?
Your body clock moves roughly an hour a day, so a 6-hour time difference can take around 6 days to settle fully. Timing light well helps you shift in the right direction; timing it badly can make the trip longer.
When should I get light after flying east?
In your body's morning, which for the first days after a big eastward flight is late morning to early afternoon at your destination. Light in the early destination morning can fall before your body temperature low and push your clock the wrong way, so keep that time dim.
Should I take melatonin for jet lag?
Low doses (0.5 to 1 mg is commonly used) taken at the destination bedtime can help some people shift. Talk to a doctor about dose and timing first, especially if you take other medication, and don't give melatonin to children without medical advice.