I was looking at the JFK-HKG flight paths for CX843 and CX831, and I noticed that they sometimes fly west and sometimes go east towards HKG. What is the reasoning for each direction? I would naively assume going west would be preferred because of the polar easterlies.

With this in mind, does the easterly bound CX843 (1:55AM departure) spend most of its flight in daylight while the same westerly bound flight would be in darkness?