Speaking as someone in the Eastern time zone of North America...
Afternoons are generally the busiest times, because that's when we can get both Europeans (evenings for them) and Americans/Canadians who either haven't got 9-5 jobs, or who IRC from work (hi, Roger!
). We've been thinking of working to encourage a later evening chat, too, but haven't actually done anything about it yet. If we do, we'll probably announce it on the boards somewhere.
PJ