1x short flight (1-2 hours) and one long flight (12 hours) and a 2 to 3 hour connection time.

That way, I can get some zzzzzzzzzzzsss on the long flight, don't need to worry about slight delays and I can make a stop at the lounge*. My recent 8h layover at LHR was sooooo boring. Spending an hour or two in a lounge is acceptable, but beyond 4 or 5 hours, I'm just waiting for the boarding to finally commence.

On two evenly split flights of 7 hours, I wouldn't be able to get some rest. On a 12 hour flight, I manage even in Y to get an hour of sleep (or 2 to 4 hours of sleep in C/F).

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* This doesn't always work. On a LH-flight in Y, the passengers in front of me were so noisy (and they wouldn't calm down), that I didn't sleep a second on the flight from HND to FRA. As a result, I was horribly exhausted by the time we arrived in FRA. Moreover, I spent the complete duration of the connecting flight (45 min in total) sleeping: Didn't notice the aircraft taxiing, taking off, flying or landing. I was rather surprised to wake up at the destination with a snack and a water in my hands.