How should night time be determined and logged in a fast westbound plane?
I don't think many airline crew actually bother to log the day/night breakdown.

At my airline we have different pay rates for day and night flying, so we do care, but the company "Electronic Logbook" tracks the day/night breakdown for us. The system is not 100% accurate and we sometimes get paid more or less day or night then we actually fly in, but it tends to average out over the year. 

In the summer, polar routes can be as much as 15 hour flights of daylight, while the same route in the winter can be 15 hours of night flying. 

On some flights we will see the sun set, and then rise, and set again, as we chase the sun and our route changes from northwest bound to southwest bound. 



Here are some samples of summer/winter, day/night Toronto to Hong Kong flying in my eLog: