I prefer to play, when the tables are full of fish, gamblers and drunks, and when most of the regs are sleeping. And for sites like PokerStars and
888 Poker this typically mean playing in the early morning hours, if you are based in Europe. From around 8-9 am CET the games tend to become pretty reg infested, and this basically last until the evening. From around 8 pm CET games tend to loosen up a little bit, but this is to late to play large tournaments, since they will play well into the night. So at this time I really only like to play cash games or small SnGs.