In my experience, day/night is not so much a factor on the NAT tracks. It is more an east/west thing. The eastbound tracks try to take advantage of the jetstreams by flying in/close to them to reduce flight times. For the same reasons westbound flights try to minimise flight times by avoiding the mainly westerly jets. Generally more turbulence is associated with the jetstream than outside of it, hence more turbulence likely eastbound than westbound.