Now, it’s true that I’m now in the same time-frame as my European tweeps; but there aren't as many of them, and some of them are late-to-rise and late-to-bed and therefore keep schedules that aren't that different than East Cost Americans.
One more factor: this whole summer I’ve been on the computer less often than usual and more irregularly.
The result of all this temporal dislocation is that when I’m online, not much is happening in my little corner of the Twitterverse — and, it turns out, browsing through tweets that are ten or twelve hours old isn't all that interesting. I look with envy at conversations that sprang up while I was away: while I could join in belatedly, that usually feels pointless. (Imagine remembering a funny joke the day after a dinner party with friends and emailing it to them.)
So it turns out that, for me anyway, much of the value of Twitter comes from actually being in the flow of it. This is perhaps why I like separating my Twitter feed from my RSS feeds: a few months ago I experimented with trying to get everything into Twitter and setting RSS aside, but I didn't like it. Whatever turns up in my RSS feed I can read later, can read whenever; but with Twitter, well, you just had to be there.