I’ve got experience of doing that sort of run to across Asia although not to Japan - mainly BKK, KUL, SIN, DPS. I usually treat it as two different flights and with 2-3 hours break in the very nice lounges in Doha to break them up. Typically I’d probably end up in Doha from 11pm to 2am or something, so had food then a nap on flight one, a few drinks in the break, then straight to sleep on flight two until I naturally woke up at which point I would probably have 3-4 hours of watching TV and eating. I mean, whatever the times it is still a bit of a grind!

The nicest bit is getting to have a shower in Doha, getting on that second flight feeling fresh makes all the different, it’’s like a total reset.