I read a very good scientific article of why that is happening to many artist - because your brain has been working through the day and the later you stay, the less your brain "worries" - aka, you make smoother lines because you do it automatically, without thinking too much, or you come up with better concepts and ideas, because your brain is not as self-conscious of how other will perceive it. In a way, staying late makes you more liberal with your art choices.
Wow that is fascinating - is that why I prefer to draw / create early in the morning before work? My brain feels completely clear at about 5am, but unlike many of you guys, I find it hard to stay up late and create unless under some kind of intense deadline...