My original contribution to this thread was largely based on gut-reaction and anecdotal evidence. However, particularly in view of Zachvac's comments, I've been given it a little more thought and attempted to apply a degree of logic to the question.
The first thing that I would say is that (for most people) I would suggest that the question is irrelevant. I do not mean that as a criticism of the question as table select is important, and therefore so to should when to play be important. However, how many people have that much choice when they play?
Many (most?) people will sleep for 7 hours, spend 10 hours on employment related issues (getting to work, working, getting home etc) and other family commitments. They will have other interests away from poker, and they are NOT going to postpone the big football game because it coincides with the most profitable poker playing time!
For those who do not need to work (or for whom poker IS their work) they will probably play significantly more anyway, so will play at the 'good' times and the 'bad' times. So, eventually to the logic.
The better players will almost certainly in most cases be the players who play most often. Some may argue with this, and there may be some exceptions, but I suspect it will generally be the case. Now, whether players are "better player" because they play more often, or whether players play more often because they are "better players"! is a whole different argument!
So, if you accept that the good players play more often, the busiest times on a poker site will be when the less frequent players play. Taking this (admittedly arguable) logic a step further, there will be a greater percentage of poor plays at the tables when they are at their busiest. So, to get a pretty decent steer on the the best times to play, all you need to do is find the times of day when there is the most traffic - so just look at the CC poker room reviews.
Well, I'm off to get my tin hat now for protection against the missiles that will be heading in my direction following this dubious logic.