For me it´s very simple:
Sieges:
Brazilian Time - 10 a.m - 12 p.m - (right before players going for lunch with their families)
CEST - 03 p.m - 05 p.m - (right after players went for lunch)
Koreans - 22 p.m to 00 a.m (not too late for players that need to work on next day)
Chinese - 21 p.m to 23 p.m (not too late for players that need to work on next day).
Olympiads:
Could stay how it is, or, maybe, considering that the majority of players today are from asia region, going back like 3-4 hours in-game for this events would be a good idea, so they would not need to play late at night, and obviously, more players would join.