New York to Paris or London is a six-hour eastward shift, taken overnight, arriving in the morning after a short night. It is the hardest jet lag most travellers meet regularly, because eastward means advancing the body clock and that is the slower direction.
New York to London is five hours; to Paris, Berlin or Madrid it is six. The plan below uses six, which covers most of western Europe.
6h eastward shift. ~4 days to full adaptation.
| Factor | This route |
|---|---|
| Direction | eastward — 6 hours |
| Severity | moderate |
| Days to adapt | about 4 |
| Harder or easier direction | Harder. Advancing the body clock is slower than delaying it. |
Left alone in the dark, the human body clock runs slightly longer than 24 hours. Flying west asks it to stretch, which is roughly what it wants to do anyway. Flying east asks it to compress, against that drift, and that is why the same number of hours costs more going east than west.
It is also why the light timing matters more on this route than on the return leg. Light at the wrong hour going east does not merely fail to help — it pushes the clock the wrong way.
The first full day at the destination, which is where most of the adaptation is won or lost.
All times are destination local time, for a traveller who normally sleeps 11:00 PM to 7:00 AM. Enter your own hours in the jet lag tool for a schedule built around your sleep rather than this example.
| What to do | When |
|---|---|
| Bright light on waking (30+ min) | 10:30 AM - 1:30 PM |
| Avoid bright light (dark glasses) | 5:30 AM - 9:30 AM |
| Target sleep window | 3:30 AM - 11:30 AM |
| Melatonin (0.5 mg) | 10:30 PM - 11:30 PM |
The instinct on landing in Europe at 8am is to get straight out into the sunshine. On arrival day that instinct is early: your body clock has not moved yet, and light before its low point pushes you the wrong way. The useful window sits later than it feels like it should, and moves earlier over the following days as you adapt.
That is the single reason to follow a schedule rather than improvise on this route. Everything else is manageable by feel.
A six-hour eastward shift is moderate, and typically takes around four days to adapt with a deliberate plan. Without one it takes longer, because the natural drift of the body clock works against you going east.
The body clock runs slightly longer than 24 hours, so delaying it — which is what flying west asks — is close to what it wants to do anyway. Advancing it, which is what flying east asks, is working against that drift.
A short nap early in the day is survivable; a long afternoon sleep is what turns one bad night into three. If you nap, keep it brief and well before evening.
This is the part people get wrong. On arrival day your body clock has not moved yet, so the useful light window is not simply "the morning" — it sits later than instinct suggests and shifts earlier over the following days.
Around four days with a plan. The first two are the ones that matter most, since that is when light exposure does the most work.