Wait, what?
Conversely, if you post at midnight, you will get more time, but the time will be less valuable. So in the end, I think it probably evens out to the same number of users visiting an article.
The trick (not that I'm a big submitter) I would guess is to post things at all hours. I imagine many people have a habbit of only cheching at a certain time. Maybe it's their lunch break, maybe just before going to bed. By posting things at all hours of the day, you can reach all of HN, rather than just the subset that visits only at certain times.
if your submissions aren't doing well, reconsider the content, not the timing ;)
It'd be interesting if we had some empirical data, though.
1) if the first one performs well, people are probably ignoring the second for being a dupe.
2) a source website's reputation, page load times, amount and type of advertising, and site design all factor into the popularity of a submission.
3) luck is a factor. just because something does better this friday night, doesn't mean that every friday night is a good time to submit. the first couple upvotes are crucial, and that's only one or two people - easily within any error margin.
4) maybe i'm totally wrong. this isn't a scientific analysis, just my general perceptions, and i've only actively been reading HN for a couple months.