Jet lag, and where a session sits inside it
Jet lag is the body clock running on the time zone you left; the direction of travel decides whether a session fits better before sleep or after it.
After a long flight across time zones, the body keeps the hours of the place it came from for a day or more. Sleep, appetite and alertness drift out of step with the clock on the hotel wall, and a traveller on a layover feels this sharply, because the stop is short and the next flight is already waiting.
Flying east
Arriving from the Americas in the early morning, the body thinks it is the middle of the night. A short sleep first, then food, then the session tends to suit. A long sleep makes the onward journey harder.
Flying west
Arriving from Asia or the Gulf in the afternoon, the body thinks it is evening. A session first, then a short sleep of about ninety minutes, keeps the stop from turning into a second night.
What helps either way
Water before the session, because cabin air dries you out. A light meal, not a heavy one. A warm room and a shower. No alcohol, which thins sleep and dries you out further. Daylight afterwards, even a short walk, to give the body clock something to hold on to.
Jet lag also affects judgement. Travellers often book a length that is too long for the stop, or too short for how tired they feel. It helps to decide the length before landing, when the mind is clearer. The jet lag page goes further, and the day room entry covers the kind of room most jet-lagged layovers use.