|Depart San Francisco at 8 AM||Arrive New York at 4:40 PM|
|Depart New York at 6:15 PM||Arrive Paris at 6:30 AM|
Let's say you leave San Francisco early in the morning and change planes in New York City. Since the connecting flight leaves New York at 6:15 PM Eastern Time (3:15 PM Pacific Time, 11:15 PM in Paris), you can begin behaving on Paris time by setting your watch to 11:15 PM and sleeping as much as possible until 6:30 AM when you arrive in Paris. This will allow up to seven hours and 45 minutes of sleep during your flight to prepare you for a full day in Paris upon your arrival.
|Depart San Francisco at 9 PM||Arrive Paris at 3:30 PM|
When you leave San Francisco at 9:00 PM it is already 5:00 AM in Paris. Attempting to reset your body clock to Paris time on this flight would force you to wake up in 2 to 3 hours to have breakfast on Paris time and subsequently suffer severe sleep deprivation. It is much better to sleep on San Francisco time on this flight and after your arrival in Paris (7:30 AM San Francisco time) switch to Paris time (3:30 PM).
As you can see different flight times result in completely different recommendations, even though the end goal is the same: to adjust to Paris time as quickly and efficiently as possible.