Data checked 2026-09-15
- Time difference—
- Strategy—
- Before you fly—
- Flight—
Where these numbers come from
Each number shows the source and the value used.
More calculators
How it works
Jet lag comes down to one moment: your body-temperature low, the point in the night when your core temperature bottoms out. This calculator puts it 3 hours before you wake up on your own (studies put it 2–4 hours before), and that's the only thing it estimates from you.
Light works in opposite directions around that point:
- Light in the 4 hours before the low shifts your body clock later.
- Light in the 4 hours after the low shifts your body clock earlier.
So the same sunshine can speed up your adjustment or undo it, depending on the hour. The calculator follows your body-temperature low day by day and tells you when to get light and when to avoid it.
From North America, you shift later
Seoul's clock is 13 hours ahead of New York's in July. That's the same as 11 hours behind plus a day, and your body clock ignores the date. So you'd expect to move it 13 hours earlier, but the calculator moves it 11 hours later: 4 days with 3 days of prep, against 8 days the other way. Flights from Los Angeles and Chicago work out the same way. From Europe the gap is under 12 hours, and shifting earlier is quicker.
Examples
Days until you're on Korea time, waking at 7:00 at home and in Korea, following the light plan "somewhat". Daylight saving time changes the gap, so each cell shows mid-July · mid-January.
| Flying from | Seoul vs. home | Fastest shift | Days, no prep | Days, 3 days of prep |
|---|---|---|---|---|
| New York | +13 h · +14 h | later by 11 h · later by 10 h | 6 · 5 | 4 · 4 |
| Los Angeles | +16 h · +17 h | later by 8 h · later by 7 h | 4 · 4 | 3 · 2 |
| London | +8 h · +9 h | earlier by 8 h · earlier by 9 h | 6 · 6 | 5 · 5 |
| Sydney | −1 h · −2 h | no plan needed · later by 2 h | 0 · 1 | 0 · 0 |
| Bangkok | +2 h · +2 h | earlier by 2 h · earlier by 2 h | 2 · 2 | 0 · 0 |
Melatonin, if you're visiting Korea
- You can't buy it over the counter in Korea. It's a prescription-only medicine there, approved only for short-term treatment of insomnia in people 55 and over (Korea's drug database, MFDS).
- Bringing your own is less clear than you'd think. Korea Customs says supplements that contain melatonin may be destroyed or sent back at customs. Korea's guidance for travelers carrying medicine asks for proof of personal use, such as a prescription or pharmacy label, allows up to 6 bottles or a 3-month supply, and recommends the original packaging. We couldn't find a rule that names melatonin in travelers' luggage, so if you rely on it, ask Korea Customs (Q&A in English) before you fly.
- Timing matters more than dose. CDC notes that melatonin taken at the wrong time can make the mismatch worse, that 0.5–1 mg is often enough, and that it doesn't recommend more than 5 mg. In the US, melatonin isn't regulated by the FDA, so the amount in a pill may differ from the label.
Assumptions
| Item | Value | Source |
|---|---|---|
| Body-temperature low | 3 hours before your natural wake-up time (literature: 2–4 hours) | CDC Yellow Book 2026 |
| Light windows | 4 hours before and after the low | Burgess 2011 |
| Speed, shifting later | 2 hours a day if you follow the light plan, 1.5 if you can't | Burgess 2011 |
| Speed, shifting earlier | 1.5 hours a day if you follow the light plan, 1 if you can't | Burgess 2011 |
| Adjusting before you fly | Moving your schedule 1 hour a day moves your clock about 60 minutes later, or 37.5 minutes earlier (literature: 30–45) | Burgess 2003; Smith 2009 |
| Short stays | 2 days or less: stay on home time | CDC Yellow Book 2026 |
| Time difference | For your travel date, daylight saving time included | IANA Time Zone Database |
From the author
I've never had much trouble with jet lag. I go to bed late even at home in Korea, so when I flew west and had to shift later, my body followed on its own. "Shifting later is easier" held true for me.
The hard part was something else: keeping in touch with people back home. My girlfriend at the time, now my wife, and I were never awake at the same time, and when I forced my sleep around our calls, my rhythm fell apart. This calculator only plans your sleep. It can't tell you when you'll be able to call home, and for me that was the harder problem. — Hotteok's dad
What this calculator can't do
- It isn't medical advice. It's general information based on sleep-medicine literature. If you have a sleep disorder or take medication, talk to a doctor.
- Your body-temperature low is an estimate. Measuring it takes temperature or melatonin tests. Worked back from your wake-up time, it can be off by 1–2 hours.
- It doesn't adjust for morning or evening types. We found no numbers to base that on. Entering the time you wake up without an alarm captures some of the difference.
- Bright light isn't for everyone. If you take medication that makes you sensitive to light, ask a doctor first.
- It doesn't show when you can reach people back home. It only plans sleep, and pushing bedtime back for calls can undo the plan.
- There's no plan for the flight home yet. The same rules apply in reverse, but you'd start from Korea time, so it needs its own calculation.
Questions
Korea is ahead of the US. Why does the plan shift my body clock later?
Seoul's clock is 13 hours ahead of New York's in July. That's the same as 11 hours behind plus a day, and your body clock ignores the date. So it can move 13 hours earlier or 11 hours later. Later is the easy direction, because the human body clock runs a little longer than 24 hours: it takes 4 days with 3 days of prep, against 8 days going earlier. The calculator works out both directions and picks the faster one.
Does adjusting before the flight really help?
Yes. Moving bed and wake-up times 1 hour a day for 3 days shifts your body clock about 3.0 hours when you're shifting later, but only about 1.9 hours when you're shifting earlier. For New York → Seoul in July, that's 6 days to adjust without prep and 4 days with it.
I'm only in Korea for two days. Should I adjust?
No. For stays of 2 days or less, the calculator tells you to stay on home time, in line with CDC's advice for short trips. You'd fly home before adjusting, then have to adjust all over again.
Can I buy melatonin in Korea, or bring my own?
Not over the counter: melatonin is a prescription-only medicine in Korea. Bringing your own is less clear. Korea Customs says supplements that contain melatonin may be destroyed or sent back, and Korea's guidance for travelers asks for proof such as a prescription for any medicine you carry. If you rely on it, ask Korea Customs before you fly.
Does it matter when I get sunlight?
Yes. Light in the 4 hours before your body-temperature low pushes your clock later; light in the 4 hours after pulls it earlier. Daylight at the wrong hour undoes your progress, which is why the plan also lists times to avoid light.