A flight across the Atlantic ends with an arrival time which is in the middle of our own biological night. Nevertheless, even though delayed, natural sleep will be possible. Our bodies are more easily able to lengthen the 24 hour cycle than to shorten it. A westward flight with a six hour prolongation of the cycle seems to present no problem. The six extra slack hours are rapidly made up with two or three slightly longer circadian cycles.
Eastward travel is a different matter.
Flights are usually overnight and only a short sleep is normally possible often before the night-time sleep gate is reached. Our bodies find it almost impossible to shorten the 24 hour cycle. We would require two days of 21 hours or three of 22 hours to compensate. Probably the best alternative is to elect for one day of 42 hours. The disruption of our cycle may take up to ten days to repair and to bring our biological clock into synchrony with local timing.