This has been questioned a lot.
Nobody has a definitive answer.
Just remember the UK is 4 or 5 hrs ahead of most of the North American timezeones and Asia is ~12 hrs ahead.
I have had posts do well in one time slot and crash and burn later, it depends on the topic, who is online, whether your post gets in front of a whale or a couple dolphins that UV it and like it...
There are just too many co factors.
Post what you want, when you want LOL, trust me this subject has been beaten to death since I got here last August!
Good question though buddy.