Odd enough, this is one of the topics me and one of my friends in SL get on often, seeing as he's over in Europe someplace and I'm in the US...so that makes for tons of fun.
But that's the thing about SL, it can be difficult to meet up with people or hang around/RP with people then they are way over there someplace. Yes, SL provides a medium for meeting the people over there someplace, but it doesn't get rid of the fact there's the pesky RL that you have to attend to. Which, yes, does include sleep. I have a tendency to be often reminded of this when I'm running around in RP or just chatting with some people, since most of the people on my friend's list somehow manage to be in Europe and such. So that's a lot of hours difference.
Though i do have to say that two of the people i spend most of my online time with...they can manage to stay up rather late. Which I think is just awesome since it actually allows me to IM/RP/whatever with 'em. So...yay!
Of course there's more than sleep added to timezone issues, too. And that would be work and school and family and friends and all the stuff that makes life, well, life. No, of course you can't just get rid of that stuff (you'd probably regret if if ya did) but that does mean you might miss someone who gets online. But, that's okay, because there is this wonderful thing called 'anotehr day'. Yes, yes there might be something you wanted to drag someone (by that I mean TP..though I do managed to get 'dragged' a lot...so maybe i mean dragged too :P ) to show that day specifically, but...while SL is forever changing, there are some things that you can still show some other time.
Basically, yes, timezones can be very evil in the fact everyone can't be working on the same time and someone might have to sleep when you've got hours to go, but...that's the thing about the Internet. Timezones stick around, but the space creating those zones? They're all but erased.