For me and my sleep habits, the best time to play poker is 6am to 6pm.
Unfortunately, this is the worst time to be playing in terms of opponent skill level.
The weaker players only come out at night. Playing at 6am, there won't be very many fish, most of the player pool is gonna be competent at worst. Think about it - your casual player isn't firing up the poker client at 6am, only us grinders and poker afficianados will be doing that...
...it shouldn't take a genius to realize the best time to play poker is the same time as when the worst players are playing, not the time when the better players make up the majority of the player pool.
So I'm in a dilemma each day. Some days I feel less tired in the evenings and I play poker, but it's hard for me to anticipate or plan it out. I rarely play when I'm 100% alert and awake, in the early mornings, for the reason I stated above. If I do get an urge, I'll often just fire up
poker stars and use the play chips from the Land of the Not-So-Free here in America. Play chips is fun for me, it costs nothing and I find the level of play at the top rungs of the play chip ladder (us play chip Billionaires on Stars) is fairly good, it's not just joke poker like many think play chip poker is. The players are playing for all the right reasons and none of the wrong ones (the wrong reason being money). Play chip poker is not the toughest game around, hardly so... but it works for me. It passes the time which is usually all I'm really doing (since money is not my reason for playing poker).